A reader submitted a link to Joshua Stephen's post about bivariate choropleths, which is the technical term for the map that FiveThirtyEight printed on abortion bans, discussed here. Joshua advocates greater usage of maps with two-dimensional color scales.
As a reminder, the fundamental building block is expressed in this bivariate color legend:
Counties are classified into one of these nine groups, based on low/middle/high ratings on two dimensions, distance and congestion.
The nine groups are given nine colors, built from superimposing shades of green and pink. All nine colors are printed on the same map.
Without a doubt, using these nine related colors are better than nine arbitrary colors. But is this a good data visualization?
Specifically, is the above map better than the pair of maps below?
The split map is produced by Josh to explain that the bivariate choropleth is just the superposition of two univariate choropleths. I much prefer the split map to the superimposed one.
Think about what the reader goes through when comparing two counties.
Superimposing the two univariate maps solves one problem: it removes the need to scan back and forth between two maps, looking for the same locations, something that is imprecise. (Unless, the map is interactive, and highlighting one county highlights the same county in the other map.)
For me, that's a small price to pay for quicker translation of color into information.