I often rant about pie charts so this post has been in the back of my mind for a while now. I just typed "hate pie charts" into Google and it seems I am not alone in this. Also, everyone else seems to express themselves far more eloquently than me:
- John Graham Cummings: "I don't like them because they fail to convey information ... people have a really hard time judging relative areas instead of lengths
- Edward Tufte: "One of the prevailing orthodoxies of this forum - one to which I whole-heartedly subscribe - is that pie charts are bad and that the only thing worse than one pie chart is lots of them" (However, he then presents an example of a useful collection of pie charts)
- A PDF report entitled "Save the Pies for Dessert" from Stephen Few of Perpetual Edge
For search engine marketing reporting and data analysis I think the most important factors are:
- When reporting you are trying to show how things change over time. Pie charts cannot do this. By presenting performance data in a pie chart you are asking your client to find your previous report so that the two charts can be compared.
- Comparing values from two pie charts is difficult; people are bad at estimating angles and areas but they are much better at estimating lengths. This is a weakness for other chart types as well, but I think it is particularly bad for pie charts.
- Comparing values within a pie chart is hard for the same reasons. If you are trying to show that quantity X is bigger than quantity Y then a pie chart is not a good way to do it
Tufte shows one way in which pie charts are useful: showing proportions together with spatial data