Reading through Tableau’s recent white paper, Which Chart Or Graph Is Right For You, I was heartened to see this explanation of pie carts:
Pie charts should be used to show relative proportions – or percentages – of information. That’s it. Despite this narrow recommendation for when to use pies, they are made with abandon. As a result, they are the most commonly mis-used chart type. If you are trying to compare data, leave it to bars or stacked bars. Don’t ask your viewer to translate pie wedges into relevant data or compare one pie to another. Key points from your data will be missed and the viewer has to work too hard.
If you have to create charts or graphs, or interpret data for work or fun, you should review this white paper.