When you're summarizing coarse relative size across arbitrary categories, bar charts and tables give your selection of categories too much weight: everyone looks at the biggest bar or the biggest number and it's really hard to measure a long tail distribution visually.

If the message you're trying to send is, "nothing but X and Y matter, everything else combined is tiny", I still can't imagine a better visualization than a pie (or a doughnut).

However, in part-to-whole relationship tasks, pie charts can outperform bar charts, since in bar charts there is no true "whole".

(In addition to Few's post, Kosara has more on this: http://eagereyes.org/criticism/in-defense-of-pie-charts)

http://www.ncsu.edu/labwrite/res/gh/gh-bargraph.html#composi...