A Bar-of-Pie Chart. And a Pie-of-Pie Chart may be even worse. Let the Chart Busters explain.
I know John’s an expert and all, and he literally wrote the book on Excel Charts. Make that, the books. And these are good books, starting with the basics and progressing into some intermediate topics.
I know the books are good, because I helped out with technical editing. I made sure the protocols and the tricks all worked, and I checked out all the sample files. I even encouraged discussions of proper chart type selection.
Disclosure: these book covers link to Amazon.com, and if you buy one after following my links, I get a commission of a few percent.
So how did John decide to present the results?
Besides the usual yuck of a pie chart, the stacked bars in the bar-of-pie chart provide even more distortion. See the bar for Guinea Pig? If the labels weren’t there, I would have thought there were more guinea pigs than hamsters, rats, and other, combined. This is a problem with encoding values in areas, particularly when the scale of area to unit changes so drastically.
By default, Excel makes the bar 75% as tall as the pie’s diameter, or 37% of the pie’s area. The label on the “other” slice of the pie tells me that the bars comprise 5 units, or 5.4% of the total, but my visual cortex is overwhelmed by the hugeness of the bars.
How about a pie-of-pie chart?
After getting over your impression of a wagon with mismatched wheels, you are misled even worse by this graph. The default “other” pie has 75% the diameter and 56% the area of the main pie. It’s obvious that guinea pigs is as large a wedge as cats, right?
Can you say “cognitive dissonance”? Sure you can. As Professor Tufte has said, the only thing worse than a pie chart is more than one of them.
Improved Chart Selection
So how do you show all elements in a single chart with a single encoding scale? Class?
That’s right, our old boring friend, the bar chart. Look, guinea pigs are nowhere near as popular as cats after all.
What if you want to show the actual counts in the chart?
Easy. Data labels add the information, without being critical to your understanding of the chart.