I have been asked this question a number of times, and being a founding member of Chart Busters, of course I know the answer. I’ve answered the question a number of times, but if I answer it here, it will become available for the ages.

I describe the problem and how to correct it. If you are really interested, I finish with an explanation of why this happens.

The Problem

Let’s use some very simple data to illustrate the problem.

Let’s make a simple bar chart.

The labels were sorted from top down in the worksheet, but they appear from bottom up along the chart axis.

The Fix

It’s easy, if tedious, to correct the order of category axis labels. Select the axis, press Ctrl+1 (numeral one), the universal shortcut in Excel for Format This Object, and in Excel 2003 the following dialog appears.

The fix is simple: check the two boxes for Categories in reverse order and Value (Y) axis crosses at maximum category.

The protocol in Excel 2007 is the same, except the dialog looks a little different. You select the same options, but they are located far apart on the dialog.

This changes the order of axis labels in our bar chart.

If you forget to make the value axis cross at the maximum category, the axis will now appear at the top of the chart. After reversing the order of the categories, the maximum category is at the bottom of the axis.

Why Does Excel Do That, Anyway?

If we use the same data to make a column chart (line and area chart, too), the labels go from left to right, as expected.

Take another look at the column chart, and note where the origin of the axis system is located. I’ve indicated the origin with a red circle.

The values start low (at zero in this case) at the origin and increase in value as they move away from the origin. The category labels start with the first one next to the origin and later labels in the list extend further from the origin.

Now look at the bar chart and consider the origin of its axis system.

The values start low (zero) at the origin and increase in value as they move away from the origin. The category labels start with the first one next to the origin and later labels in the list extend further from the origin. Just like in the column chart.

Perhaps this is better illustrated if we remove the category data from the bar chart. In this case, Excel uses the counting numbers 1, 2, 3, etc. in place of the empty categories.

Both axes have low numbers next to the origin and higher numbers further away.

The whole problem arises because Excel follows the same axis ordering scheme for bar chart category axes as for any other axis in any other chart.

This describes the mechanics of axis label ordering. But, 99% of the time, a user expects the axis labels to go in the same order top to bottom as in the data source. Why Are My Excel Bar Chart Categories Backwards? is still a valid question: Why can’t bar chart categories automatically be reversed? Alternatively, why can’t the options for a bar chart’s category axis default to:

Works for me.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

The specific question was about labels 1, 2, 4, 8, etc. This kind of progression can be linearized by a logarithmic transformation; since the specific labels are powers of 2, base 2 logarithms are an exact fit.

The trick with a fake axis is that the data values are subjected to the same transformation as the axis label values, so the data lines up with the axis. The original axis label data is used for the labels themselves.

Excel 2003

We’ll start with this sample data. B2:C9 is the original data. D2:D9 has the base 2 logarithms of C2:C9.

Data for the custom axis labels is in B11:D18. Column B has zeros, for the X position of the axis points. Column C has Y values selected for the Y axis labels. Column D has the base 2 logs of these values, for positioning of the axis labels.

Here’s what the original data in column C looks like The Y position is proportional to the Y value.

This is the transformed Y data in column D. The Y position in the chart is proportional to the logarithm of the Y value.

We can’t leave those Y axis labels there. People can’t readily convert 0, 1, 2, etc. into 1, 2, 4 in their heads. Most people, anyway. Select B11:B18, hold Ctrl, and select D11:D18, so that both areas are selected. Copy, select the chart, and use Paste Special to add the data as a new series, with categories in the first column and series names in the first row.

Add the labels to the new series. I’ve used Rob Bovey’s Chart Labeler, which is one of the best free Excel utilities on the internet. The range C12:C18 contains the labels, and they’re added to the left of the added data points. I’ve colored the labels to match the points.>

Format the real Y axis to hide the labels, but in this case we can keep the axis line and tick marks. Adjust the width of the plot area to make room for the labels.

Hide the added series by removing lines and markers, and the chart is done.

Excel 2007

Excel 2007 makes the chart much easier, because Microsoft introduced the ability to use a logarithmic axis scale with any base. All we need is the original data. (In general the protocol would be the same for both versions, but the log transformation makes this case different.)

Here’s the data in a default XY chart.

Here’s the data with the default base-10 log axis scale.

Here is the chart with a base 2 log axis scale.

This more flexible log axis is one of the rare examples of Excel 2007 charting actually improving on Excel 2003’s charting.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

In Stupid Chart of the Day, James Joyner points out that Clarke’s chart starts from 28%, not from 0%, and thus it is deceiving. (I don’t think it’s really that stupid a chart, but maybe Joyner is going for more pageviews.) In Charts can be Deceiving E.D. Kain followed up on this observation, and produced his own chart of the data; I have reproduced Kain’s chart below.

So which chart is right, and which is deceiving? Well, both plot the data correctly, in terms of not using wrong data. But both have built-in flaws which distort the reader’s interpretations.

Using an axis that starts from zero is important in bar (and column) charts, because the visible length of the bars is what the reader sees and relates to the values. If the bars are truncated, their true length cannot be known, and the user is misled. Since the region under the line of an area chart is shaded, it implies that the area is the important encoding feature in the chart. Thus, starting the axis above zero is misleading for area charts as well as for bar charts.

I don’t think Clarke was attempting to mislead. When you enter his data into Excel and draw an area chart, Excel decides to start the axis at 28% by default, and Clarke simply made no attempt to change the default axis scale. (He did adjust the default fill by introducing a gradient, but the judges are not concerned with artistic impression here.)

And what of Kain’s chart? How is that chart deceptive? Let’s ignore the irrelevant two decimal digits he has added to his Y axis labels. Kain has started his Y axis at 0%, according to best practices for bar (and area) charts. He has made the Y axis maximum 100%, however, which compresses the data into the bottom portion of the chart. The variability in the data is dwarfed by the magnitude of the Y axis.

In any case, a line (or XY/scatter) chart has no implicit requirement to start at zero, since the position of the data points is what encodes their value.The benefit of using a line chart is that you can match the Y axis scale so that it spans from a little bit below the lowest data point to a little bit above the highest.

The 28% default minimum assigned by Excel isn’t even far enough from zero. I’ve used a minimum of 30% and a maximum of 37% on the Y axis of my line chart. This shows the steady decline in tax rate since the mid-1990s, but in no way implies that the tax rate now is 1/6 of it’s peak value.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

We’ll use the same data as in the original post.

Make a column chart using the data in columns C:E, ignoring the axis labels for now.

Format the axis to hide the default axis tick labels (but remember the sequence: 1, 2, 3, etc.).

Here is the data for the dummy (hidden) series that will provide our labels. The first block is for the puter labels: the X values are in column G, corresponding to the slots in the category axis which were numbered for us in the first chart above; the Y values (zeros) are in column H; and the labels are in column I. I’ve kept all of the rows in this first block just so the labels were aligned and I could use simple formulas.

The numbers 2, 6, and 10 in column G refer to the slots that these labels will be centered under; if any of the groups had an even number of points, we would have had to center the outer labels between two axis slots, by using an X value of (for example) 9.5. Since we’ll be making XY series of these sets of data, we could use non-integer X values if necessary.

The labels use a simple formula to insert a line feed before the text of the original label; the line feed positions these outer labels below the inner labels. The formula in cell I2 is

=CHAR(10)&A2

and this is copied down as needed. It can be used in all cells, since a line feed plus an empty string will not look like anything in the chart.

The second block, for the inner labels, is configured in the same way as the first.

Add the first dummy series by copying G1:H12, selecting the chart, and using Paste Special to add the data as a new series. The new series is not visible in the chart, because it is added as another stacked column with values (heights) of zero.

Select a visible series in the chart, and use the up arrow until the added series (“Outer Labels”) is selected. Right click on one of the selection indicators, and if the context menu has an item called Format Data Series, choose Chart Type, and select the XY type with markers but no lines. The series is indicated below as large red squares.

Excel helpfully placed the XY series onto the secondary axis. Select the XY series, press CTRL+1 (numeral one) to open the Format Series dialog, and change its axis to Primary. See how well it is aligned.

Now add data labels to this series. The easiest way is to download and install Rob Bovey’s Chart Labeler. The Chart Labeler is free and easy to use. It installs a submenu to the Tools menu, as shown below (in Excel 2007 it adds an item to the Add-Ins ribbon tab).

In the dialog, select the series, select the range containing the labels, and select the position of the labels.

Here is the chart with the outer labels.

Add the second series. Copy K1:L12, select the chart, and use Paste Special to add this data as a new series. Since the previously added series was converted to an XY series and moved to the primary axis, this chart is conveniently pasted as an XY series on the primary axis, indicated by green diamonds below.

As above, use the Chart Labeler to label the Inner Labels series.

Here is the chart with all category labels showing.

Hide the XY series by formatting them to show no lines and no markers.

Select an individual label (the first click selects the series of labels, the second selects the label itself), and format it as desired. When one label is selected, you can use the left and right arrow keys to move to adjacent labels. In Excel 2003 and earlier, the F4 key will assign the last applied format to the selected label.

Here is the chart with its category labels formatted to match the plotted data. The color used for the font is a darker variation of the fill color used for the columns; it looks like these could be even a bit darker.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

Dick wondered how to avoid this problem. There are two ways, one very simple and the other slightly simple but more attractive.

I’ll illustrate with the following data. The “opposite” data in column C is dummy data for a hidden series which will provide data labels that replace the default axis labels. The values are defined by a simple formula: =-B2/ABS(B2). I could have used =-SIGN(B2), but I thought of this variation after I built the data range.

I created a bar chart using the first two columns of data.

Since Excel plots data starting from the axis and moving away, the first category (alpha) is plotted next to the axis, below the next category. It’s actually very logical, but people don’t understand this. Anyway, fix this by formatting the category axis so it is plotted in reverse order, and the value axis crosses at the maximum category, which is  at the bottom of the chart when categories are plotted in reverse order.

Okay, looks fine, except for the label “gamma” which overlap its data point.

I promised an easy way, so here goes. Format the category axis so the axis tick labels are in the “Low” position. They are plotted at the low side of the chart. This is a good quick way to move the labels out of the way, but in some cases, the labels may be moved pretty far from the axis. Because the labels need a margin, the entire chart is compressed laterally, reducing the resolution of the value axis scale.

So here is the almost as simple method, which eliminates the issues raised above. Extend the chart source data to include the “opposite” column of data.

Format either of the bar chart series so the overlap is 100%. Now the bars line up perfectly.

Format the category axis to remove the tick labels (select the “None”) position.

Add data labels to the “opposite” series, using the Category Labels option.

Format the data labels so they are in the “Inside Base” position.

Now hide the “opposite” series by formatting it to show no border and no area fill. Also remove the “opposite” legend entry by single clicking twice on its text label, and pressing Delete. (You could remove the legend altogether, since there’s but a single series.

The technique works just as well with column charts.

You could also use the Invert Excel Chart Format if Negative technique to further highlight the negative value.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

Andrew commented that the X axis should have years centered between the tickmarks, since a year is a span of time, not an event.

I decided to demonstrate how to produce a date scale axis with centered year labels in Excel. Using the following simplified data, I reconstructed the chart of Elkins and Sides as an Excel line chart.

The year labels have a number format of “YYYY” so only the four-digit year is shown. If the months and days are also shown, we see that the year labels actually appear at the start of each year (which is evident if we examine the data).

To “fix” the labels, I will add a second line chart series, move the original series to the secondary axis, and only display the primary axis year labels. Below is the data used for this second series, as well as the chart with this series added. Although the X values (category labels) are different for this second series, in a line chart, all series in each axis group use the same labels as the first series in the axis group.

Move the original series to the secondary axis (secondary axis group) by choosing Secondary in the Format Series dialog. Note that both series now use the X labels of the second series.

When you move a series to the secondary axis group, Excel usually provides a secondary value (Y) axis but not a secondary category (X) axis. Delete the secondary Y axis and add a secondary X axis. This axis appears at the top of the chart, overlapping with the title.

Adjust the scale of the secondary date axis, so it starts at the beginning of 2004 (i.e., January 1) and ends at the start of 2008.

Finally, hide the secondary category axis by changing all of its formatting properties to None. Also hide the second series by chaning its line and marker properties to None.

Without too much muss and fuss, we’ve created a nicer date scale axis, with the year labels centered within the corresponding years, rather than at the start of each year.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

The user’s data looks like this (I’ve changed the labels and values).

The waterfall chart looks like this, after minor modifications.

The user wanted the secondary axis to scale from 0% at $0, to 100% at the top of the revenue bar, or $840,000. The simplest way to do this is to set up a table like the one below, which contains the primary axis maximum and minimum, the primary and secondary axis values which have to line up, and spaces for the secondary axis maximum and minimum.

I changed the view to show formulas in the cells, by holding Ctrl and clicking the button with the tilde (~). The formulas in C12 and C14 are really straightforward once you’ve set up a table like this.

I’ve switched back to normal view, by again holding Ctrl and clicking the ~ button. The calculated secondary axis scale parameters are now shown.

In order to have a secondary axis in an Excel chart, you need a series to put onto the secondary axis, in addition to any series that belong on the primary axis. The waterfall chart has a bunch of series on the primary axis, but we’ll add a new series for the secondary axis. Right click the chart, choose Source Data, and click on the Series tab. Click the Add button, enter a name (“Dummy”), and for values, enter ={0}, which is an array consisting of the single element zero. This results in a series which should remain hidden.

This new series is added to the primary axis. Select the new series, by using the Chart Menu’s Select Chart element dropdown, or simply by selecting any chart series (actually, any chart element) and cycling through all chart elements until the Dummy series is selected.

Once the Dummy series is selected, press Ctrl+1 (numeral one) to open the Format Series dialog. On the Axis tab, click Secondary. The chart now has a secondary axis.

Format the secondary axis: right click on the axis and choose Format Axis. On the Scale tab, enter the numbers calculated above. If necessary, adjust the number format of the axis labels on the Number tab.

You should also lock in the primary axis scale parameters, because any change Excel makes under Auto mode will break the calculations made above. Right click the axis, choose Format Axis, and on the scale tab, uncheck the Auto boxes in front of Minimum, Maximum, and Major Unit.

The result is a chart with percentages that line up as desired.

An alternative might be to scale the secondary axis from 0% to 100%, and the primary axis from $0 to $840,000.

Peltier Technical Services, Inc., Copyright © 2010.
 
Licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
 

When we calculate the percentage change as in the table above, we see that the middle green curve for electicity has actually risen more slowly than the other two fuels, and the natural gas curve isn’t far behind heating oil in terms of relative increase.

Nicolas goes on to discuss absolute and relative increases, and uses changes in the data (point-to-point changes or slopes) to make his point. HIs point is well taken, that the absolute linear scale on the vertical axis leads to incorrect conclusions. Rather than calculate a lot of slopes, and present the data in different terms than the original chart shows, there’s an easier approach: use a logarithmic value axis. I recreated Nicolas’ table, then connected the endpoints in a linear scale chart and a logarithmic scale chart.

The logarithmic scale on the right preserves the price values, while converting the slopes from absolute changes (where heating oil dwarfs the other two fuels) to relative changes (where electricity shows the slowest rise).

The use of a logarithmic scale is nothing new, of course. Financial data has long been presented on this basis, which facilitates comparisons between a $20 stock and a $30 stock, without requiring the reader to perform mental calculations.

This concludes the theoretical portion of today’s post.

How to make a nice logarithmic scale

Excel allows you to convert a linear value axis to a logarithmic value axis. Simply check the Logarithmic Scale box near the scale parameter settings. The result is okay (below left), but far from perfect. Up through Excel 2003, the axis could only begin, end, and show labels on a power of ten; Excel 2007 allows you to change the base of the log scale, a real improvement, but the labeling options are still limited to powers of the base (below right with a log base of 2).

So how did I get a nice log axis scale above? I rolled by own axis. Here is the data I used. In cells B6 and B7 I inserted “good” endpoints for my scale; I used worksheet cells to make it easy to adjust the calculations. I computed logarithmic values in E2:F4 using this formula in E2 and filling it into the range:

=(LOG(B2)-LOG($B$6))/(LOG($B$7)-LOG($B$6))
 

I put “good” tick labels in C10:C16, calculated the corresponding axis values in B10:B16 using this formula in B10:

=(LOG(C10)-LOG($B$6))/(LOG($B$7)-LOG($B$6))
 

and I put the year 1990 into A10:A16. When I construct a dummy XY series for my new axis, A10:A16 will contain X values, B10:B16 Y values, and C10:C16 data labels for the series. The value of 0.5 in cell C9 will control the length of tick marks (i.e., error bars) on this dummy series.

I selected A1:A4, then held Ctrl while selecting E1:F4, so both areas were highlighted, and created an XY chart with series in rows. These are the trends I want to show, but the axis scale does not show the actual prices.

I copied A9:B16, selected the chart, and used Paste Special to add the data as a new series, by column, with categories (X values) in the first column and series name in the first row.

I added custom positive horizontal (X) error bars to this new series, using cell C9 for the custom error bar value.

I hid the series by formatting it to show no line and no marker.

I formatted the error bars to use medium gray to match the axis line color, and to draw just the bar without the end cap.

I hid the Y axis tick marks and tick labels.

Finally I added data labels to the dummy series, using the contents of C10:C16. You can do this by adding any data labels and manually changing the text of each, but I use Rob Bovey’s free Chart Labeler for this. John Walkenbach makes a similar utility, Chart Tools.

This nicely drawn logarithmic scale chart easily shows relative prices as well as relative changes in price.

There are advantages to both sorting techniques. The sorting by occurrence (left), as in a Pareto chart, makes it easy to see which item has the largest or smallest incidence, and if you need to take a particular action, you can simply start at one end of the list and work your way to the other.

Often when people have plot this kind of data, instead of sorting by incidence, they will sort in alphabetical order. This is generally not so useful, because alphabetical sorting is rather arbitrary: only the incidence has a quantitative meaning.

Dates, however, have a quantitative meaning, an order all their own. You can use a Pareto sort, as John has, or a date-order sort, as I have. If you are concerned only with the absolute incidences, then the Pareto sort makes sense. If you are interested in the distribution of incidences over your range of dates, then the date-order sort is more useful. It might help you see that certain days of the week or months of the year tend to have higher incidences.

Another advantage of the date-order sorting (or another numerical sorting in a histogram) is that, if you are pressed for space and compress the chart, you can leave out category labels without deleting information from the chart. In the chart below right, I know that 17-Apr fits right between 16-Apr and 18-Apr. In the incidence sorting below left, I can only guess where 17-Apr fits (between 15-Apr and 30-Mar), and if the incidences changed, I would have to guess again where to find 17-Apr.

I couldn’t think of a good way to plot the way that dates move around as the sampling size of Easter dates changes. I made this table showing the incidences of the date of Easter sorted by occurrence, for 100, 200, 300, 400, and 500 years. I highlighted some arbitrary dates, showing how they move around. Some dates stay relatively close to the same position, but some move up and down many rankings. This rearrangement of the scale makes any comparisons meaningless.

In contrast, the dates in the date-order stay in the same order. By definition. I can plot the curves for 100 through 500 sample points together, and see that they follow roughly the same distribution, and I can plot the curves separately and see the same thing. Even though there may be some movement from one curve to the other, it is easier to understand the charted behavior.

Another reason why I was interested in the distribution by date was for a comparison of the dates of Easter calculated by Western Christians (i.e., the Roman dates) and those calculated by Orthodox Christians (i,.e., the Greek dates). This is part of a topic for an upcoming post, but I’ll illustrate the utility of the date-order scale here.

I can plot the two Easter date distributions either together (first chart below) or in separate panels (second chart below. I can see that the distributions are similar in shape, but offset by about two weeks (the Greeks place Easter about two weeks later on average than do the Romans). I’ve displayed only every third date, but I don’t need the missing ones to understand the distributions. A comparison of two Pareto charts would yield no meaningful observations.

In this post I will illustrate the behavior of the different category types in more detail.

Categorical Data

The following data has a column of category labels for X and a column of numerical values for Y.

The default Line and Column charts are shown below. The categories are completely visible in the chart, because the value (Y) axis is set to cross the category (X) axis between categories, the default setting for line and column charts.

Below, the value (Y) axis is set not to cross the category (X) axis between categories, so the first and last categories are only halfway visible.

The default Area chart is shown below left. Unlike the Line and Column charts, its default is for the value (Y) axis not to cross the category (X) axis between categories. When the value (Y) axis is set to cross the category (X) axis between categories, there is a gap between the edges of the filled area and the edges of the chart (below right).

A default bar chart has its category (X) axis along the vertical axis, and the bars extend horizontally parallel to the value (Y) axis. Note that the vertical axis is the X axis and the horizontal axis is the Y axis, unlike the convention in most charts. This is a common confusion for newcomers to Excel’s charts.

Here is an XY chart made using text labels for its X values. An XY chart has a value-based X axis, like its Y axis. Excel cannot plot nonnumerical data in a chart, so instead of the text labels, Excel inserts counting numbers, 1 for the first label, 2 for the second, etc. Nonnumerical values used for Y values in any chart are treated as zeros.

Numerical Data

The following data has a column of numerical values for X and a column of numerical values for Y.

Here are default Line and Column charts made using numerical values for their X values. The category (X) axis treats the X values as labels, despite their numerical character, so along the X axis points are spaced equally, not spaced according to the numerical values.

Here is an XY chart made using numbers for its X values. Of course, this is what XY charts were designed for, so both the X and Y numbers are plotted along the X and Y axes according to their numerical values.

Dates

The following data has a column of date for X and a column of numerical values for Y.

The default Line and Column charts using this data are shown below. Unlike the treatment of text or numbers as nonnumeric category labels, the dates are treated as numerical dates, with the spacing between points proportional to the number of days between points. By default, the X axis stretches from the first date to the last, and Excel has chosen a “nice” X axis spacing of 7 days (one week).

Here is an XY chart made using dates for its X values. Like the Line and Column charts shown above, the spacing of points along the X axis is proportional to the date value of the X data. The axis scale parameters are chosen as for any other value axis. The minimum, 12/29/2007, is day number 39445 in Excel’s date-time system*, the maximum, 2/7/2008, is day number 39485, and the spacing is 5 days. The formatting of the Line chart’s date-scale axis is much nicer.

* Excel stores dates and times as a mixed number: the whole number part is the number of days since January 1, 1900, and the fractional part is the fraction of the day elapsed since midnight (e.g., noon = 0.5, 6 pm = 0.75).

Weekly Dates

The superiority of Excel’s date-scale axis for a chart’s X axis is better illustrated with a broader range of dates. This table shows Friday dates for the first three months of 2008.

The default Line and Column charts using this data are shown below. The X axis stretches from the first date to the last, and Excel has chosen a “nice” calendar-related X axis spacing of 14 days (two weeks). The column chart illustrates a “problem” people have with using column charts with date-scale axes. The columns are very thin, and there is no way to make them much wider. Even changing the gap width to zero, which makes adjacent columns touch each other, leaves a large gap. This gap is created by the date-scale axis itself, which provides a slot for each day along the axis. Weekly data then consists of one data point followed by six blank slots before the next data point. Since line charts are generally a better tool to illustrate time series data, this is just one more reason not to use a column chart in this context.

The corresponding XY chart is shown below. Its X axis scale is not calendar based, starting on Monday, 12/24/2007 (day 39440), and ending on Wednesday, 4/2/2008 (day 39540), with a tick spacing of 10 days.

We can clean up the Line chart’s X axis by removing the year and rotating the resulting labels (below left) and fix up the XY chart’s X axis in the same way, and also by incorporating week-based scale parameters (below right). The charts are essentially identical.

If we format the date-scale axis to show months, the Line chart does so nicely, with a tick mark at the beginning of each month, regardless of the number of days in the preceding month. The XY chart comes up short, because we have to use a “best-fit” major unit, in this case, 30-1/3 days. January is represented in two tick mark labels and February not at all along the XY chart’s X axis.

Taken over a whole year, with a “best-fit” major unit of 30-1/2 days per month (366 days/12 months), the XY chart (shown under the Line chart, below) is even less appealing. January is shown in two tick mark labels, February not at all, and three months (March, May, and July) on the second of the month, not the first.

It is for this reason that I will often use a Line-XY combination chart: I get the benefit of the Line chart’s nice date-scale axis without the Line chart’s requirement that all series use the same X values.

Axis Types (Chart Options)

The axis types shown above for the Line, Column, and Area charts were assigned automatically by Excel, based on the X values data. You can actually tell Excel which type to use. For example, in your chart of stock prices, Excel plots every value by date, and leaves gaps in the chart for Saturdays and Sundays. You can omit the gaps by forcing Excel to use a Category type axis instead of a Date-Scale axis. In Excel 2003 and earlier, you can select Chart Options from the Chart menu, and the Axes tab of the resulting dialog lets you select which axes to use in the chart (in Excel 2007 these options are available on the Format Axis dialog). In the Line chart’s dialog (below left), the three options Automatic, Category, and Time-Scale (correctly renamed “Date-Scale” in Excel 2007) are available for selection. In the XY chart’s dialog (below right) these options are visible but disabled.

—-

In the following pairs of charts, the left hand one uses the Automatic (Category) axis, while the right hand one uses the Date-Scale type. The left hand charts, repeated from above, treat numerical X data as nonnumerical categories. In the right hand charts, the numbers in the X data are forced to be interpreted numerically, and plotted proportionally, not uniformly. The right hand charts are horizontally compressed because Excel leaves room for the longer axis tick labels which may extend beyond the plot area. The dates begin with day 1 (January 1, 1900).

The Line chart (above right) can almost be converted into an XY chart, by changing the X axis number format from a date format to a numerical format (such as “General”), setting the value axis not to cross between categories, and choosing appropriate scale parameters. Of course, it’s not perfect; since the first day in Excel’s date-time system is 1/1/1900, the smallest value that can be used as the X axis minimum is 1, not 0.

This technique comes in handy when combining an XY chart with an area chart, to fill the area under or between XY chart series.

In the following pairs of charts, the left hand one uses the Automatic (Date-Scale) axis, while the right hand one uses the Category type. The left hand charts, repeated from above, plot the dates in their X values proportionally. In the right hand charts, the dates are plotted uniformly, not proportionally.

This technique is useful to plot stock data while omitting gaps for weekends. Frankly I prefer the time scale axis; the gaps where the weekends occur give me a better sense for the date values.