Peltier Technical Services - Excel Charts and Programming
A reader named Martin suggested that I take a look at The Slate Bailout Guide, which sported a concentric set of rings purporting to be a chart. It was interactive and all, very slick. The graphic contained data regarding the economic bailout efforts by the US government over the past year. But it tried to do too much, and ended up being confusing.
[Read more…] about Hide Your Data in an Interactive Onion Chart
David Boyle of InfoClarity shows us a chart of declining interest rates in Interest rates racing down. He got the chart from Paul Kedrosky‘s The Interest Rate Race, and Paul apparently got the chart from the Wall Street Journal‘s Fed Weighs Its Options as Europe Cuts Rates (don’t bother following this link unless you have a subscription).
Click on the chart to see it at full size
Of this chart, Paul said, “Good graphic in the WSJ today showing the race to zero by central banks in major countries worldwide.” David added, “There was a nice little chart in the Wall Street Journal yesterday showing clearly how central banks are racing interest rates towards zero.”
My first impression upon seeing tha chart was, “Wow, the rates sure are declining fast!” But I was misled by the effective advance PR and the poor chart design.
Let me explain this deception by first recreating this chart.
The titles of the two posts (“The Interest Rate Race” and “Interest rates racing down”) combined with the decreasing top series of points instantly caused me to mentally connect these points, like this:
Wow, look at that decline. It was pretty steady for a while, but lately it has accelerated. See it? Here, let me help.
What could be more clear? I’m sure many people stopped here, thinking that the chart had made its point clearly.
But that’s a false conclusion to draw from this data. When you step back and let (un)common sense guide your reasoning, you will realize that the chart above does not plot a decline in interest rates over time as it may seem. It plots a decline from one country to another at one point in time in October 2007.
The data could just as easily be sorted by interest rate not at the beginning of the period, but at the end, that is, last Thursday. This is arguably more relevant, since it’s more recent. You still see a steady decline in rates, but not as steep just because of the scaling of the chart.
It’s still going down, but not as sharply.
One could plot in order of increasing change in interest rate, which implies no false trend. It does not really make a point or show anything particularly clearly.
Or in order of increasing proportional change in rate, on a logarithmic Y axis to scale the proportional change more clearly. Again, no implied pseudo-trend, but also no clear point.
If we want to observe the decline in rates more clearly, we should use a “Bumps Chart”. You can read about bumps charts at Process Trends, at Junk Charts, or at XL Cubed. Essentially bumps charts show changes in a number of related values from one time point to another (or sometimes over a series of time periods). The chart below shows the changes in interest rates from a year ago October to last week. This bumps charts shows the decline in rates clearly, for each country, without resorting to displays of exaggerated pseudo-slopes to magnify the effect.
Some time ago, I showed how a column or bar chart could display a table of data more effectively than four pie charts (or a donut chart) in Column Chart to Replace Multiple Pie Charts. I showed how to build a panel chart to plot the same data in How to Build a 2×2 Panel Chart. In this post I’ll demonstrate why donut charts are such an awful way to try to present data.
Donut Charts
This is the data used to make the donut chart above, and it’s served us for several other exercises. Rows add up to 100%.
Here again is the donut chart. You can compare values by comparing the included angles, except that only Engr1 and Mktg2 have a common baseline for all points in those categories. If you consider consider the Mktg1 (blue) sections. Three of the four have values between 19.7% and 20.0%. Without consulting the table above, it’s mighty hard to tell which doesn’t fall within that range.
Of course, we could apply data labels that display the values, but to make them fit, some have to be rotated. In this chart I was lucky, because I could stick to horizontal or vertical orientations, and not any of the pixel-squashing inclinations in between.
It’s still hard to remember which concentric arc refers to which series. Let’s change the labels to show series names. Okay, that’s better, but now I can’t remember the percentages.
The dialog lets us add both to our label, in fact, we could also add the categories from the legend. But the labels are already overcrowded. I suppose if we wanted to, we could find a package that lets us wrap the labels around an arc, but I’m glad Excel doesn’t offer that option. But at least we have all the data visible in the chart that was in the original table of data, only not as easy to read.
Donuts, an Exploded View
I’ve pivoted the data so all values are in one column. I’ve also calculated the area of each segment, with a scaling factor that conveniently equals 100 for the hole in the center. The innermost arc has a total area of 300 (that is, three times the area of the central hole), the second 500, the third 700, and the outermost 900. If you’ve been listening to me for a while (I mean months, not just this post), you can guess where this is going.
To start this analysis I had to explode the donut chart. Quick, call Dundas, a new chart type! This was easier than I had expected: I copied the donut as a picture in Excel, pasted onto a PowerPoint slide, and ungrouped twice. Then I dragged the pieces into position.
Then I arranged the segments in order of ascending value from top to bottom. As you can see by eyeballing the pieces, or by looking at the Area column or dot plot in the accompanying sorted table, the area jumps around a lot as the value monotonically increases. If a measure such as area is to be a reliable indicator of value, both measure and value should increase monotonically and proportionally.
Here is the same set of building blocks, this time sorted by area. We can tell from the table, but not from the arcs themselves, that value jumps up and down a lot as area increases.
Bar Charts
Let’s compare the donut’s area-value lack of correlation above with the area-value characteristics of a bar chart. This is the data and chart from Column Chart to Replace Multiple Pie Charts.
Here I’ve removed the gaps between clusters and inserted a narrow white strip between adjacent bars. I’ve put labels in the bars so the legend isn’t necessary to identify anything.
Then I sorted the values and rearranged the bars accordingly. The bars are sorted by value and by length. Since each bar has the same thickness, they are also sorted by area.
Comparison
Which shows the better correlation between value and measure of that value?
In You are NOT spider man, so why do you use radar charts? Chandoo presented a “petal chart” as an alternative to a radar chart (or spider chart, after its spider-web-like appearance). Here is the radar chart Chandoo wanted to replace:
And here is the replacement petal chart:
That’s a pretty jagged flower. I think the name “petal chart” probably refers to an idealized chart of this type:
Though the corresponding radar chart is somewhat meaningless:
It actually seems that the petal chart is even worse at allowing comparisons than the lowly radar chart. At least in a radar chart, the items being compared are all along the same spoke, whereas in the petal chart, they are distributed around the circle, 90° apart in this example. Also, the false zero values on either side of the categories for each petal can be distracting.
Radar charts are not very effective, despite their frequent appearance where scores in a set of categories are being compared. Their radial nature gives more emphasis to larger values toward the outside of the chart. If there are any zero values, the chart can become very difficult to read:
This bias toward larger values and the confusion caused by zeros can be minimized by selecting a more appropriate chart type, the familiar line chart:
The line chart is a better choice even if the radar chart has no zeros to confound the data:
A vertical line chart (or dot plot) is also a good choice:
Derek of Information Ocean has analyzed Chandoo’s radar and petal charts in Reorderable tables II: Bertin versus the Spiders. He applied Jacques Bertin’s assertion that charts are really just tables, and after a little rearrangement of the categories and series, came up with this spot matrix:
Derek rearranged the data to impart a diagonal trend to the chart. He likes spot matrix charts, but I’m not really wild about them. Bertin liked to use a stack of column charts to show this type of data, and I think I prefer Derek’s stacked chart to his spot matrix.
Shrink the stacked chart a bit, and it becomes a sparkline.
The radar chart is no better for ordering of the data:
The line charts and dot plots are improved somewhat by the ordering. The sorted data makes it easier to see that the yellow line (option 1 in Derek’s charts) is good in all areas except for usability (probably the worst category to rank poorly in). The blue line (option 2) has outstanding usability, but it’s at or near the worst in four of the other five categories.
Bottom line: Radar charts are not too effective for what they are often used for. Chandoo made a noble attempt to improve on them, but sticking to a chart with a circular shape wasn’t a good move: the petal chart looked nice, but it was even less effective at displaying the data. Line charts are one of the best choices, though Derek presented a couple good alternatives. Derek’s sorting of the categories helps to bring out patterns in the data, in his charts and in the line charts.
The Dashboard Spy wrote about Animating Dashboard Charts with Flash on the Dashboards by Example blog. The examples in the Spy’s post were taken from our friends at Fusion Charts:
As I discussed in my comment to the Spy’s post is that a little animation goes a long way.
If “animation” only means slick effects showing the graphics being constructed, then it is a waste of resources and of the user’s time. A good dashboard does not need animation. The graphics should appear instantly in their completed state. [Read more…] about Animated Dashboards?