This is, as you have heard repeatedly, the most divided Congress in history. There are various metrics that are used to calculated that division, but National Journal's annual vote ratings, released for the new Congress in February, offers as good an overview as you might need to prove the point. "Welcome to today's Congress," Josh Kraushaar wrote, "which in 2013 was more polarized than any Congress since National Journal began calculating its ratings in 1982."

That's true. The National Journal system measures voting records over the course of the year in an attempt to figure out which of his or her peers each member of Congress compares most closely to. For the fourth year, Kraushaar wrote, no Senate Democrat was more conservative than a Republican in that body, and vice versa. The middle ground has washed away.

But we wanted to see that erosion. So we pulled NJ's ranking data for each senator since 1993, and plotted their voting record on a scale from most to conservative to most liberal. Compare the 103rd Congress, in 1993, with today's. That purple zone in the 103rd Congress was where Republican and Democratic ideologies mixed, the left edge marked the most conservative Democrat and the right edge the most liberal Republican. There's no purple zone anymore.

One way to see how the Senate has changed is with a graph like this, with the more faded dots representing older Congresses. (Larger dots depict new senators; the colors, of course, break down by Democrat and Republican.)

But we wanted you to really be able to explore the data. So we made a tool that will let you step through the data, one Congress at a time.

One interesting thing to look at is how senators likely to face tough reelection campaigns often move toward the center. (What a surprise!) We pulled data for five incumbents facing tough reelection races. In each case, their positions in 2013 were closer to the opposite ideological pole than in the 111th Congress.

It will be interesting to see what happens to those who win reelection -- if they feel comfortable in moving back away from the center, at least until five years from now.