It would be helpful to see an animation to clarify this, but it's interesting to quantify how increasingly polarized this has become, starting in the early '90s.
+1 If the author could get data from 1980-2013, and make it into a 60 second animation, it would be really interesting to see.
I've definitely considered that, but it's computationally difficult to make it in the browser. This animated gif is as close as it comes: http://i.minus.com/ibi7p66VbETpnU.gif
This is illuminating, also would be if you could add years to each slide that would be even more fascinating. Great work and thanks for sharing!