The primary strength of the pie chart is the obvious message of the “part-to-whole” relationship in visually appealing way that can be appreciated by all (see Choosing the right chart type). Bar charts with stacking and/or proper labeling can also achieve this message but in a slightly less intuitive manner. Beyond this small advantage, however, it’s often argued that pie charts are the least effective form of graphs (e.g. on Wikipedia or in Edward Tufte’s “The Visual Display of Quantitative Information”).
The size of elements in pie chart can be fairly easily estimated when the element begins at 0, 90, 180, or 270 degrees. If, however, it starts at a different angle the size of the element becomes much more difficult to estimate. In the first example below, the blue element is easily approximated at 25%. In the second example, when the chart has been rotated, the size of the blue element is more difficult to estimate.
The eye can quickly compare differences location and line length (as used in bar charts) but has difficulty discerning differences in angles and areas (as used in pie charts). In the first three examples below, you can quickly arrange the elements from largest to smallest. In the third example, however, it takes a little longer to ascertain that that D is larger than B.
Furthermore, it’s easier to extend bar charts into Mekkos where you can quickly compare the width of each bar. To extend the pie chart you have to create multiple pie charts where the total area or diameter is used to compare data sets; a comparison that is not always easy to make. In the first example below, you can quickly discern that the bar W is about 5 times wider that than bar Q. The second example, however, it is much more difficult to determine that pie W is 5 times the area of pie Q.
Pie charts can be used to effectively display very simple data in a visually appealing style. However, if the data has any complexity at all, forgo the stylistic points and opt for the more effective bar graph.