Pie charts are evil in the same way that the devil is evil: through mental trickery they beguile you, clouding your judgment while giving you the sense that you are making perfectly rational decisions. Pie charts obscure the very data story that they are meant to tell. And that – for visual data representation – is truly evil.
Everybody knows that pie charts look great – especially 3D! – and no doubt there are some situations in which they can be used to get the message across. But they come up short on a critical component of data visualization: context. The only context present within a pie chart is the relationship of individual data points to each other. But this is usually only part of the picture. To gain an understanding of what is going on in our data universe, we almost always need to be able to monitor trends over time. And this is where pie charts fall apart.
It is interesting to know, for example, the distribution of website traffic among various sources, as represented in a pie chart. But this is not a static relationship – it is likely to be changing and the important thing is to be able to understand how it is changing.
At a presentation to reviewing the progress of search marketing efforts, one slide included the following chart:
All well and good, provided out-of-the-box by Google Analytics so it is easy to grab, and it gives a snapshot of where things stand. But it tells us NOTHING about any progress that has been made (or not) in driving traffic from search engines to the site. Not quite as sexy, but your basic bar chart can tell us what has been happening much more effectively. Here are a couple different bar chart options, depending on the aspects of the story that you want to emphasize:
Bar chart on the left shows percentage breakdown and now we can a) clearly see the relationships, while b) observing that traffic from Search Engines and Referring Sites is growing, as a percentage of total traffic, at the expense of Direct traffic. Bar chart on the right shows us more clearly the degree of change from one month to the next in each of the categories. Now we can see what’s going on and base our decisions accordingly. Each one takes up about as much space as the pie chart, yet delivers much more valuable information.
More often than not, when you’re tempted to use a pie chart, a less glamorous visual tool may do the job better. This is why hard-core data visualization masters such as Edward Tufte and Stephen Few (.pdf link) renounce pie charts in all but very particular circumstances. And so should we all.