I was not aware of this xkcd comic, funny how things seem to pop out. My inspiration was more in the line of Sankey diagrams. Among others, the famous map of Napoleon's invasion of Russia by Minard (http://en.wikipedia.org/wiki/File:Minard.png). I've called this representation a "groupflow" but any name would do.

The notion of agreement departs a little bit from the classical fraction of common votes/total votes and instead is based on a vector model of votes, using cosine similarity as a value of agreement. The agreement groups are computed by heuristically optimizing a quantity called the cohesion and which turns out is really correlated to the way people perceive social communities (the old preprint of the first paper: http://arxiv.org/pdf/1107.3231.pdf).