It's a truism in politics that Congress has become increasingly partisan. But rarely have we seen that reality displayed more bluntly than in the chart below -- created by the good folks at xkcd.
Think of the chart as a tree. There are three branches designated for each party: center right (light red), right (red) and far right (dark red) for Republicans and center left (light blue), left (blue) and far left (dark blue) for Democrats.
What the chart does is trace those three branches -- in the House and Senate -- from the late 1700s to present day. And what is immediately clear is that the once-dominant "center" roots in both parties have shrunk considerably, a shrinkage that began in earnest in the 1980s.
The center in the House has withered more than the center in the Senate. And the center in the GOP has disappeared faster (and more completely) -- particularly in the House -- than the center for Democrats.
Here's a bigger version of the chart. And scroll down below the chart to see a blown-up version of the legend that explains -- in a more detailed way -- what you are seeing.