Believe it or not, this dataviz illustrates a good use of pie charts. I know, right? It shows the proportion of DC Metro passengers who are entering and exiting during the morning peak hours. And the pie charts work - best, I'll argue - because there are only two slices AND because the data, generally, as a natural order from greatest to least as one moves toward the center. An alternative graph type, like a bar chart, would not have the same immediate understanding at a glance.
Here's the original source: http://greatergreaterwashington.org/post/16811/which-metro-stations-are-the-most-balanced/
-SE
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Having only 2 classes in the pies works very well to provide an overview of the flow. A small improvement might be to match the pie chart color with the Metro line color, which would help differentiate them. -RS
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I love this chart because it resonates with people from Washington, DC so well. When I share this chart with DC friends and colleagues, we immediately start pointing out our respective starting and ending metro stops and telling horror stories about our metro delays and hour-long commutes. There's something about this chart's simplicity that pulls the readers in and starts a conversation.
I also like exploring the chart for the half-and-half pie charts, that is, the metro stations with an equal number of people getting on and off the metro, like Ballston, near my own apartment. I've always envisioned Ballston as a sort of metro hub where people are busily coming and going, and the graphic confirmed some of my hypotheses. -AKE