The popularity of pie charts is a bit of a mystery. The data visualization goals of a pie chart can easily and more effectively be shown with a bar chart or even a simple table. In the worst of cases, a whole pie chart is used to show a single number:
That’s an example pulled from Google Analytics by Coda Hale. He’s justifiably flabbergasted that "this piechart uses 78,050 pixels to display a single fact—that 9.94% of all visitors had previously visited the site—resulting in a spectacular data-point-to-pixel ratio of 0.0013%."
Pie charts break some of the basic rules of data visualization:
- Readers of pie charts will struggle to judge the relative size of the pie slices. If the goal is to clearly show your results, pie charts can do more harm than good.
- As noted, pie charts aren’t particularly efficient in the data-to-ink/pixel ratio. They take a lot of space to present a little information.
- A well-designed pie chart is hard to pull off—colors need to have high contrast, labels need to be carefully placed to not overlap and be readable, too many values quickly result into a visual mess.
- The all too popular 3-D pie chart exacerbates all of the above.
Here’s a little more color commentary from Coda and others to elucidate this problem:
In his post about Google Analytics, Coda Hale takes out some aggression on pie charts. A few of my favorite quotes:
Piecharts are defeat in circular form.
Piecharts are for middle management.
Piecharts are the information visualization equivalent of a roofing hammer to the frontal lobe.
[Piecharts] have no place in the world of grownups, and occupy the same semiotic space as short pants, a runny nose, and chocolate smeared on one’s face. They are as professional as a pair of assless chaps. Anyone who suggests their use should be instinctively slapped.
To my mind, the best use of a pie chart is when you have one value that is overwhelmingly larger than the rest and you don’t want the audience to focus on the actual values, but just bamboozle them with the overwhelming size of the leading segment. Of course, this seems to come close to embracing the old adage, "There are lies, damn lies and statistics."
Wikipedia’s entry on pie charts even includes a warning against using:
Pie charts are rare in the scientific literature, but are more common in business and economics. One reason for this may be that it is more difficult for comparisons to be made between items in a chart when area is used instead of length. In Stevens’ power law, visual area is perceived with a power of 0.7 compared to length that is 1.0. This implies that length would be a better scale to use, since differences would be linearly related. Pie charts should be used only when the sum of all categories is meaningful, for example if they represent proportions.
In research at AT&T Bell Laboratories, it was shown that comparisons using angles was less accurate than comparisons using length. This can be illustrated with the diagram below. Most subjects have difficulty ordering the slices in the pie chart, however when a bar chart was used the comparison is much clearer.
Maybe I’m being too hard on pie charts. Consider, for example, their unique ability to support a joke (via BoingBoing):
Try pulling that off, Bar Chart!