Time to Cut the Pie
Folks who've worked with me are well aware that I have two seething hatreds I nurse: micromanagement and pie charts. Micromanagement I'll rant about some other day; today let's bury pie charts.
I know, I know---some of you think they're cute and harmless. They are neither. In a world drowning in data and thirsty for information, pie charts are an oasis. As nice as you may think they look---and given all the 3D effects, dropped shadow, and bizarre gradients many incorporate into their presentation, you're trying very hard to tart them up---they serve only to obscure real insight and make us all just that little bit stupider than we were before we saw them.
There are a number of issues with pie charts:
- Our brains do not process relative differences in conical areas well. The whole point of a pie chart is to compare the relative mix of elements. This is why you often resort to labeling the wedges, which forces your audience to follow the clock around comparing numbers.
- Pie charts do not display rank order clearly. As the relative size of each wedge narrrows, can you tell which is larger simply from its position? Where is the starting point of the pie, which is, after all, circular?
- Pie charts struggle with numerous wedges. The more wedges you have, the harder it will prove to distinguish between them. Available area for labels shrinks too, leading to those charts with the legends and values called out somewhere to the right.
- Pie charts are slow to interpret. One of my favorite tests of an information graphic is speed to insight. Grab a stopwatch, put a graphic in front of someone, ask them a question, hit Start, and then hit Stop after they give the correct answer to the question. Time doesn't lie.
- Pie charts are difficult to interpret with small multiples. Sometimes we like to put lots of little charts together for comparison purposes. If you wanted to look at, say, employees by degree achieved across various departments of your company, there would be a lot of squinting and scanning back and forth.
There is a simple alternative to pie charts which is readily available, always included in the same software that allows you to whip up those pies. It's called a bar chart and it is superior in every way to the pie.
- In interpreting a bar chart, our brains need only process comparitive lengths of the bars. Guess what? Our brains are really good at doing this.
- Bar charts are sorted from most to least or least to most. Rank order is therefore very clear.
- You can fit many bars in a small space. Essentially, the only limit is the font size of the axis label.
- Bar charts are quick to interpret. Take your data and make a pie and a bar chart out of it. Put each in front of the same person (wait a few days in-between in case they memorize the answer) and ask them your question. Time doesn't lie.
- Bar charts work well in small multiples. The one caveat here is the more bars you have, the smaller the label will be. Applications like JMP allow you to select the bar in the chart and see the relevant row of the data set highlighted, which sidesteps this issue nicely. The bars can also be color-coded to help highlight differences.
As an example, here is the same data shown with a pie chart and a bar chart.
Note that the extended axis in the bar chart is there to demonstrate that no one color dominates this scenario; it serves the same purpose as the circumference of the pie.
The prosecution rests.