And this doesn’t even discuss the effect of changing the color scale on the visual appearance (linear vs. log vs. some binning algorithm etc.), measuring counts

And this doesn’t even discuss the effect of changing the color scale on the visual appearance (linear vs. log vs. some binning algorithm etc.), measuring counts per region vs. per capita vs. per square mile, drawing a cartogram, etc. There are a lot of choices when displaying data on a map using colors. Mike & Jeremy: one that you might want to toss in here is normalizing so that regions matching the national average increase are colored neutrally (instead of no change being colored neutral). i.e. normalizing relative to the national trend instead of relative to the previous year’s value. Depending on the data, this can help separate national vs. regional trends, though it takes more careful explanation to avoid reader confusion.