Often we see groups of charts in which the individual charts are fine but the group misleads. In other cases, the individual charts have problems and the groupings make matters worse. The problem with the groups might be inconsistent colors, scales, order or other graphical elements.
In Figure 1 the meanings of the colors in the map and the accompanying pie chart are not consistent. A reader looking at the map will see that black is used for Kentucky. The reader might assume that the black on the pie chart refers to Kentucky without reading the labels carefully. However, the black on the pie chart refers to Piedmont. Consistency of color is important in groups of graphs.
The graphs in Figure 2 were discussed in Bill Clinton’s Speech in Misleading Graphs as examples of misleading bar graphs without a zero baseline. Another problem with these figures is that the scales are not the same. A reader might compare the heights of the bars in the two figures without noticing that the scales differ.
In Figure 3 both the revenues and the profits of these Fortune 500 companies are ordered by size. The reader might read the labels of one of the charts and assume that the order of the other is the same. This is a case where two principles of effective graphs contradict one another. The first principle is that alphabetical ordering is rarely best and that ordering by size is often more informative. The second is that groups of charts need consistent ordering. My advice when two principles conflict is to choose the principle that is least likely to confuse or mislead the reader. In this case I suggest ordering one of the variables by size and using the same ordering for the other. In this case I would order revenue by size and use the revenue ordering for profits as well.
In the preceding example we said that scales must be consistent and here they are not. There are times when the range of values in each of the charts makes consistent scales impractical since most of the variation in one of the variables would be lost. We used the caption to call attention to the fact that the scales differ.
When drawing charts and graphs it is important that the individual charts be clear, concise, and easy to understand. In addition, we must consider consistency in groups of charts to make sure that we do not confuse or mislead the reader.
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