Within the past year, I’ve led 60+ in-person and virtual workshops for 2,800+ participants. Most of these trainings have focused on data visualization best practices and how-to’s; other topics have included dashboard automation, research methods, and data analysis.
I can always tell when someone has attended a data visualization training in the past because they tell me, “Ann! I know everything there is to know about data visualization! I know that I can never use pie charts!”
That advice about never using pie charts is only half-true.
Pie charts are okay when:
- they’re well-formatted (meaning that they’re 2D, not 3D; there aren’t any exploding slices; and there aren’t any distracting leader lines or legends);
- they’re displaying nominal or categorical data;
- they add to 100% (not 99% or 101% due to rounding irregularities or 110% due to crazy lapses in judgment);
- they contain positive numbers (not negative numbers);
- they’re showing a single point in time;
- they’ve only got 2 to 3 different slices; and
- there’s only one pie chart shown at a time (i.e., not a small multiples layout).
Finally, while I don’t consider this to be a strict guideline, pie charts tend to be easiest to read with common fractions, like a one-fourth vs. three-fourths pie or a one-third vs. two-thirds pie.
The chart on the lower left is poorly formatted. This one is 3D, so the slices look larger or smaller than they really are… and, it’s exploding, which is distracting for viewers… and instead of the percentages being right on top of the pie slices, now there’s a tiny legend down below the pie, which means our viewers would have to zig-zag their eyes around the slide to tell which slice is which. The final sin in this poorly-formatted pie chart is that there are leader lines, those gray lines connecting the 25% and 75% to their corresponding slices. Plus, So much ink is on the page, yet so little is actually focused on the data.
The well-formatted pie chart on the lower right is fair game. Gender is nominal or categorical, so that works. We’re only showing a single point in time, so that works too. And we’ve only got two different slices.