Stacked Bar Charts
The preceding section discussed some commonsense guidelines for redrawing a sample exhibit produced using Excel's default settings. The chart format used in the example was the venerable bar chart—a format most readers should be familiar with.
This section introduces a variation on the bar chart—the stacked bar (or column) chart. When comparing categorized data across multiple time periods, stacked bar charts are a reasonable choice, when two conditions hold true:
- You need to analyze more than two time periods
- The number of categories being compared do not exceed about a half dozen
Two variants of the stacked bar chart exist—one that normalizes all values relative to their percentage share of the total, and one that does not. Readers of The Economist should recognize the former. Figure 6-4 shows a sample "normalized" stacked bar chart created using a spreadsheet package.
Figure 6-4 Stacked Bar Chart (Normalized)
To increase readability, I tweaked the chart as follows:
- Grouped all data rows after the top six into a catchall category called "Other."
- Removed the background fill and all tick marks.
- Simplified the y-axis labels so that they show only the minimum (0) and maximum (100%) values.
- Manually reversed (and put in boldface) the series labels that lie on dark-colored backgrounds.
- Stretched the chart legend vertically to better align the legend labels with the bar positions. Note how the chart legend items line up, horizontally, with the items in the stacked bars.
- Manually added an opaque white background to the "3%" label.
- Changed the color of the "Other" category to white (so that it is less noticeable than the named categories).
- Tweaked the color scheme so that the shades of gray alternate between light and dark (to improve contrast between categories, and thus readability).
Figure 6-5 shows the same data, but plotted using the second, non-normalized stacked bar chart variant.
Figure 6-5 Stacked Bar Chart (Nonnormalized)
I do not use stacked bar charts often, but they can be useful when the number of data series and corresponding points are relatively small. The primary advantage of stacked bar charts is that readers recognize them fairly readily.