Lots of us in visualization like to berate Pie Charts, primarily due to their constant abuse and mis-use in mainstream media. Honestly, how many powerpoint presentations have you sat in with a pie chart showing 15 or 20 slices, most under 5%? Robert Kosara has a great article up referencing a paper from 1991, “Displaying Proportions and Percentages” in Applied Congnitive Psychology, that analyzes the usability of Pie Charts compared to Tables and Horizontal Bar charts, and finds them not just acceptable but in fact better (under a very specific set of circumstances).
While it is not surprising that the table doesn’t do too well in the more complex tasks (adding up numbers is more difficult than combining areas), the comparatively bad performance of the bar chart is. They did not include the stacked bar chart here for some reason, which presumably would have done better than the regular one.
In their discussion, they mention that adjacent segments in a pie chart are especially easy to combine, whereas the same is not true for a bar chart. And that, I think, is an important point: if we step away from the focus on accuracy in reading numbers, some of the more complex tasks are actually easier to do with the pie chart. The bar chart is great for reading and comparing individual numbers, but when it comes to adding up bars, it’s a lot less effective.
