Data visualization experts will often show pie charts as an example of “bad” visualization. Xan Gregg (one of my colleagues at JMP) has posted the challenge “One Less Pie” for pie day, to show some better ways to visualize data.
But, you may ask, what’s wrong with pie charts?
One of the first questions we asked in that survey was “Of the books you purchased last year, what percent were e-books?” We could show the results using a pie chart, like so:
However, the pie chart loses the ordering. It’s not obvious in the resulting chart that “Less than 20 Books” is ordered next to “20 – 50”. A better visualization, like a share chart (or a stacked bar), shows the proportions without losing the ordering:
This gets more important as you add information to the visualization. Maybe we want to see if people who play video games buy more, less, or the same number of e-books as people who don’t. Which visualization would you prefer for that question?
Or Share Chart/Stacked Bar?
I prefer the stacked bar graph because it lets you compare the proportions across groups easily and quickly. The information is more compact, and it communicates a lot more than scanning across four different pie charts.
There are places where a pie chart can work. For example, the “Do you want cool stuff?” question has a small number of levels, and there’s no real order to them. A pie chart communicates who said “yes” and who said “no” pretty easily: