I had an impulse to visualize the common intuition that progressive politics is positively correlated with population density. The result is this visualization, built on d3.js. It shows two linked views of per-county data: a map and a scatterplot.
Understanding the visualization
Each circle in the scatterplot is a county; size represents population., color represents degree of Republican-leaning (red) vs Democratic-leaning (blue) votes in the 2012 presidential election.
The x-axis of the scatterplot is the population density (people/km^2) on a log scale. The y-axis is % of the vote that was Democratic in the 2012 presidential election.
Try clicking and dragging to select a region in the scatterplot or map.
The correlation between density and ideology is pretty obvious. Some interesting clusters pop out from the visualization, such as the small set of places that are low-denisity but Democratic, which appear to be rural areas with a racial minority population.