Randall Munroe has outdone himself. XKCD, the "webcomic of romance, sarcasm, math, and language," just posted another amazing, wall-sized infographic, this one depicting the historical ideological swings of left, right and center of the US Senate and House of Representatives (here are Randall's other *huge* and hugely fascinating infographics).
Be sure to read the side boxes and especially the one on methodology of how ideology was calculated. He meticulously accounts for the historical shift in the left/right spectrum between Republicans and Democrats.
That is all.