I got lots of feedback on Twitter about my post yesterday about using using area or radius of circles to show numeric values (on a map in this case). The consensus seems to be that radius greatly exaggerates the differences, while area somewhat under-represents the differences. So area seems like the better choice if you are going to use circles to represent the data (and it’s time for me to reheat some humble pie, again!).
Ever-helpful Chris provided a couple of pointers that I wanted to share. The first is the use of “perceptual” scaling, ie start with area but then make some adjustment to account for the fact that people can’t perceive differences in area very well:
There’s more details here, although the exact adjustment to make seems to be an empirical question where the answer varies across people (ie it is a bit more Art than Science).
Another suggestion of his:
— Chris McDowall (@fogonwater) March 12, 2015
That could solve the problem quite well, although it requires the reader to pay good attention to the legend to interpret the data.
All up, it seems to be somewhat tricky to show numeric data like this on a map, because a map is a two-dimensional thing and so you end up wanting to use an extra dimension to represent the data than the data has. If it’s useful to map the data (rather than just drawing a bar chart), my starting point now would be to think about whether just putting the numbers on the map would be adequate.
Editor’s note: we continue to work through Aaron’s excellent back-catalogue of posts.