Isarithmic map of presidential election data from every election between 1920 and 2008. Created by David B. Sparks, a PhD candidate in political science at Duke.
Sparks explains on his blog, “Isarithmic maps are essentially topographic or contour maps, wherein a third variable is represented in two dimensions by color, or by contour lines, indicating gradations.” In this case, the third variable is partisan intensity, represented by shading.
http://ecopolitology.org/2010/11/23/stunning-animated-time-lapse-map-of-red-blue-america-1920-2008/