This is an example of using a [clever color key] that doubles as a histogram showing the distribution of the corresponding areas.

By the way, this post is from a great [series] about small ways to make better visualizations.

[clever color key]: http://www.visualisingdata.com/2016/04/little-visualisation-design-part-14/ “Little of Visualization of Design: Part 14”

[series]: http://www.visualisingdata.com/2016/03/little-visualisation-design/ “All the ‘Little of Visualisation of Design'”