Pies, bars and self-sufficiency
Mar 29, 2021
Andy Cotgreave asked Twitter followers to pick between pie charts and bar charts:
The underlying data are proportions of people who say they won't get the coronavirus vaccine.
I noticed two somewhat unusual features: the use of pies to show single proportions, and the aspect ratio of the bars (taller than typical). Which version is easier to understand?
To answer this question, I like to apply a self-sufficiency test. This test is used to determine whether the readers are using the visual elements of the chart to udnerstand the data, or are they bypassing the visual elements and just reading the data labels? So, let's remove the printed data from the chart and take another look:
For me, these charts are comparable. Each is moderately hard to read. That's because the percentages fall into a narrow range at one end of the range. For both charts, many readers are likely to be looking for the data labels.
Here's a sketch of a design that is self-sufficient.
The data do not appear on this chart.
My first reaction to Andy's tweet turned out to be a misreading of the charts. I thought he was disaggregating the pie chart, like we can unstack a stacked bar chart.
Looking at the data more carefully, I realize that the "proportions" are not part to the whole. Or rather, the whole isn't "all races" or "all education levels". The whole is all respondents of a particular type.
The one thing that sticks out to me is that on the bar chart, I'd estimate both "black healthcare" and "less than college" to be roughly equal and roughly 20%, while on the pie charts it's immediately obvious one is > 25% and the other is below.
Ordering the bar charts might fix the comparison, but pie charts do seem to give good reference points to multiples of 25%.
Posted by: Robert | Apr 02, 2021 at 05:45 PM