Pie charts are the worst.
Well, at least, they way they seem to be used most of the time is. By-on-large they accomplish the exact opposite of what good visualisation should be. That of simply conveying big or complex data.
That being said, they can easily be misused on even the most simple data, for example:
What we see here is what should be simple data horrendously used in pie chart form. Whilst clearly most visits in December came from fellow EU citizens, could you see whether the UK had more visits from North Americans or other European citizens which do not have EU membership? Right there is the inherent problem with pie charts, humans are pretty horrible at distinguishing relative polygon sizes. So why we insist we rely on them I have no idea.
Techcrunch posted a particularly horrible example way back in 2011:
Now, brushing over the fact that the author has given up on labeling the data points (which look awful by the way) on the last few categories. It is all but impossible to compare the sizes of the segments. Something the labeling seeks to address by adding the percentage values, but by doing so defeating the point of displaying the data in the first place.
A chart that uses this much data should never, ever resort to pie charts. Especially when it’s something that a good old bar chart can display much more effectively and the tourism figures are a great example.
What was once blurred, is now clear. The use of grid-lines demonstrates that non-EU visitors just topped North American tourists last December.
Now if there’s one thing worse than pie charts, its 3D pie charts. Once again techcrunch has a tremendous example:
Not only do we have the aforementioned problems of the previous chart but now this unspeakable error is compounded by the unfathomable choice to tilt the chart. Now, the ‘web’ section is made to look a whole lot bigger than it actually is because an author thought it would look cool or something. The chart is supposed to show market share of twitter apps but any ability to distinguish percentages is completely erased by the ‘3D-fication’ of the darn thing.
(NB: when finding out how to save excel charts as image files to post here I manged to stumble upon a step-by-step which used 3D pie charts as an example, it was painful)
I could go on posting countless examples of bad use of pie charts but its probably worth ending on when they can be useful. When comparing just one thing to another, that’s it. For a great example, Chandoo.org has one: