The Wikipedia page for Pie Charts now has a nice section illustrating how bar charts do a better job of visualizing differences. This comparative chart (created using R, see the code here) makes the point clearly:
On a related note, someone called Omegatron is on a humanitarian mission to rid Wikipedia of uninformative pie charts. You can see his accomplishments (1,2) at Junk Charts.
Junk Charts: Community outreach: pie-making
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