A history of Congress from XKCD...dang. This is really good!

This should be hanging on the wall in every classroom.  It was created by the web comic XKCD.  Blow it up big on your screen, it's worth reading all the tiny print.

Here's what's really interesting: the categories into which congressmen are sifted (left,right, far left, far right)  are not primarily based on the expressed political philosophy except in a very generic mostly one-dimensional (a bit more detail on this below) sense, or on the content of bills they vote for or speeches they make.  The groups are based on a particular clustering index, on how often people vote together.  So in a way, each Congress should have its own separate index.  But since Congressmen come and go and overlap in their times in Congress, each group forms a chain of like-minded people from the beginning of the country.  But that doesn't mean that the people in any chain now would agree on every issue with those who were in the same chain 200 years ago---this is kind of like a game of telephone, I think, with each generation coming along with slightly different views, but forming fairly solid blocks with those who occupied a similar ideological space in the cohorts just before them, and voting with them much of the time.

On the categories, I won't give too much detail because I'm still trying to absorb it myself, but here's a quote from the Wikipedia entry devoted to the DW-Nominate scoring method, which is what XKDC used:

"Poole and Rosenthal demonstrate that—despite the many complexities of congressional representation and politics—roll call voting in both the House and the Senate can be organized and explained by no more than two dimensions throughout the sweep of American history. The first dimension (horizontal or x-axis) is the familiar left-right (or liberal-conservative) spectrum on economic matters. The second dimension (vertical or y-axis) picks up attitudes on cross-cutting, salient issues of the day (which include or have included slavery, bimetallism, civil rights, regional, and social/lifestyle issues). For the most part, congressional voting is uni-dimensional, with most of the variation in voting patterns explained by placement along the liberal-conservative first dimension."

The emphasis is mine, but I thought I’d point out that the primary  explanation of the vote clustering is the Congress member’s feelings about economics, and specifically about how much the government should intrude on the nation’s economic life.

Absolutely a fascinating graphic.  It's worth staring at for a while.  XKCD is worth staring at for a while.  With this graphic is has become a whole lot more than just a web comic.  Or maybe it has expanded the web-comic genre into a much bigger space.