What's an "isarithmic visualization"?

According to David Sparks, it's basically a map "wherein a third variable is represented in two dimensions by color, or by contour lines, indicating gradations." Hemade one tracking the two-party vote since 1920, and it's oddly beautiful. Remember that this is tracking change in the vote, not the total vote. So when the map turns red or blue, that means that Republicans or Democrats are gaining voters, not that they necessarily won those areas.

From Ezra Klein