“Death to the pie chart” is a battle cry of the data visualization expert Cole Nussbaumer working in the bay area mainly with business clients. I learned a lot about visual communication of data from her blog, after all, it does not matter if plots are about business revenue or bacterial growth!
While pie charts are not outright wrong, they are very, very hard to read accurately. In my classes I ask students to estimate the percentage of categories read from either a pie chart or a bar chart – invariably, they do better with getting the information from the bar chart!* And very often, the error when reading from pie chart is 10% or more! Would you want to force your audience, be it your busy boss, readership of your paper or of the next grant, to have to make guesses about your numbers or rather be sure they easily grasp it from a well executed bar chart? The longer it takes your audience to understand your figures, the less likely he/she will be to want to continue reading.
I have an exercise for you! In the pie chart below, is blue or red bigger? How long did you take to figure it out?
Now double-check your result using the bar chart – and monitor how much faster it was reading it! And there you already have the answer: in the pie chart you actually have to guess while in the bar you will be able to pretty precisely read of the answer from the provided axis!
And, surprise surprise, if I turn the bar chart by 90 deg, the category names are right next to the bar and we can read names with the bar length just like we read text. This helps the overall readability tremendously and is of course way more interesting if categories have more complicated labels such as “Wildtype animal treated with DDSX5 (5mM)”.
The real numbers are A = 18%, B, C, E = 20%, D = 22%. Please comment how close you got, I am collecting the data, after all I am still a scientist!
Come back for my next blog on a re-vamping a real example and also how to nicely show percentages with bar charts!
PS Cole’s blog on pie chart can be found here
* of course, in each class is one person who is really good at reading pie charts and this invariably leads to a lot of discussion. It is wonderful for that person to be good at pie charts, but it still shows that most people have difficulties reading from pie. Since figures are targeted at as many people as possible, I recommend making the figures as readable as possible to as many people as you can! And this includes, do not use the pie chart!