In Line-XY Combination Charts I showed how to make a combination Line-XY chart. It is probably important to discuss the differences between line charts and XY charts. The documentation is not clear, and the names of the chart types are not helpful; in fact, they lead to confusion.

The icons in the Chart Type dialog and in the Chart Wizard do not help to clarify the situation. These are the icons for line charts and XY charts in the Excel 2003 dialogs; the Excel 2007 icons are not substantially different.

It is important to note that:

• The formatting options of XY chart series and line chart series are identical.
• If you want markers connected by lines, you DO NOT have to use a line chart type.
• If you want markers without connecting lines, you DO NOT have to use an XY chart type.
• XY and line charts treat X data differently and thus have different X axis styles.

After selection of a chart type, the user is presented with a set of choices, and the default sub-type for the selected chart type is highlighted. Not all available formatting for these chart types is available through the Chart Type dialog.

Line Chart and XY Chart Sub-Types

Here are the sub-types for a line chart. The first row shows options with lines but no markers. The second row shows options with lines and markers. The third row shows a 3D “ribbon” version of a line chart; you should NEVER use this abominable chart type. Among the first rows of sub-types, the first column shows each series plotted as their actual values. The second column of sub-types shows the series stacked upon each other, and the third column of sub-types shows the stacked values, normalized to the total at each category, that it, normalized to 100%. You should avoid using stacked line chart sub-types, because they cause confusion both for those making charts and for those interpreting charts.

Here are the sub-types for an XY chart. The first column of icons show sub-types with markers, the second column shows those without markers. The first row shows the sub-type with no lines, the second row shows sub-types with smoothed lines, and the third row shows sub-types with straight lines. You should avoid charts with smoothed lines, especially without markers, because the smoothed lines may misrepresent the actual data being plotted.

Line Chart and XY Chart Formatting

To illustrate the identical formatting possibilities of XY and line charts, let’s compare the Format Series dialogs. This is the Format Series dialog for a line chart series:

This is the Format Series dialog for an XY chart series:

The Patterns tab is identical, both chart types offer the same line and marker formatting options. What is different is that the line chart offers a tab for only Y error bars, while the XY chart offers tabs for both Y error bars and X error bars. The two chart types offer a different set of options on the Options tab, but those are not important to this discussion.

Line Chart and XY Chart Axes

The difference between a line chart and an XY chart has nothing to do with formatting of the series plotted in the charts. The difference relates to how the X data is treated by the chart. There are two important* types of axis in an Excel chart:

• Category (X) Axis - Independent variable, usually the horizontal axis.
• Value (Y) Axis - Dependent variable, usually the vertical axis.

*Some 3D chart types also have a Series axis, which is essentially an additional Category-type axis. Since 3D charts should rarely be used, this axis type is rarely worth worrying about.

There are three varieties of axes in an Excel chart, defined by how the chart uses the X data. There is some duplication of terminology with the axis types described above, but you should try not to mix the chart axis types (above) with these styles.

• Category Axis - Data is treated as non-numeric text labels, regardless of any apparent numerical values in the data.
• Date-Scale Axis - Data is treated as numerical integer dates. Inaccurately called a “time-scale” axis in Excel 2003 and earlier, and renamed “date-scale” in Excel 2007.
• Value Axis - Data is treated as continuously varying numerical data.

The Value (Y) axis in every Excel chart is a value axis. The Category (X) axis in an Excel XY chart is also a value axis.

The Category (X) axes in most Excel charts (line, column, area, bar) can be either Category or Date-Scale axes. If not specified, Excel automatically chooses based on the data. if the data contains time or date formatting, the Date-Scale style is applied; otherwise the Category style is used. The user can overrule Excel’s automatic selection.

Numerical X Values
Below are two charts created using regular numerical data, a line chart (left) and an XY chart (right). The line chart places the points uniformly along the X axis, using the numerical values as non-numerical text labels. The XY chart plots the points according to actual X values.

Text X Labels
Below are two charts created using text labels as X values, a line chart (left) and an XY chart (right). The line chart places the points uniformly along the X axis, using text labels as axis tick labels. The XY chart doesn’t know how to interpret the labels numerically, so it uses the counting numbers {1, 2, 3,…} as X values, and it uses these numbers, not the text labels, to label the axis.

Dates as X Values
Below are three charts which use dates for X values: a line chart with a date-scale axis (left), a line chart with a category axis (center), and an XY chart (right). Both the line chart with the date-scale axis and the XY chart space the data horizontally according to the dates, while the line chart with the category axis spaces the data uniformly along the X axis. Note that the date-scale axis has a slot for each date along the axis, whether there is a corresponding point or not; the category axis only has categories for dates that have Y values. Vertical axis labels like these aree difficult to read and should not be used in practice; I’ve used them only to illustrate features of the different axis styles.

The intricacies of these chart axes will be covered in upcoming posts. This article should at least clear up the confusion between XY and Line chart types.

Possibly Related Posts:

• Excel Category Axis Types
• Integer Values on Line Chart Category Axis
• Order of Points in XY and Line Charts
• Category Axis Tricks for Line and Area Charts - 2
• Line-XY Combination Charts
• Category Axis Tricks for Line and Area Charts - 1
• Marimekko Charts
• Connect Two XY Series
• Individually Formatted Category Axis Labels
• Clustered-Stacked Column Charts with Vertical Separators