The Washington Post has a good idea. Using Census data, they computed the proportion of police force who are white and the corresponding proportion of citizens who are white, in different cities.

In the following scatter plot, they singled out North Charleston, SC where the police force is 85% white but the citizens are only 40% white: (Link to the interactive chart.)

This plot itself is well done, with helpful coloring and labels.

One must be careful about "story time": it's easy to infer from the graph that blue dots mean worse racial tension but that interpretation requires an assumption not proven in the data. (What is missing is the correlation between this data and some other data measuring tension.)

The secret to reading this chart is to look at the slopes of lines from the origin to each point. Above the 45-degree diagonal separating the blue dots from the gray are the cities where the police is more white than the people. The steeper the line to the origin, the more unrepresentative. Once you pass the 45-degree line, do the reverse.

The slope is really the metric of X police per Y residents. So the two dimensions can be collapsed into one. With the one dimension, I'd try a histogram view. If you find the data, let me know. Or just post it to the comments.

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