Here's the visualization in GIF form.
The images tell a striking story. As recently as 1989, the U.S. Senate looked a lot like a purple ball -- meaning that there was a large amount of cross-party voting and similarities in voting records between members of different parties. By 2013, a blue ball and a red ball -- with almost no boundaries touching -- had emerged.
This visualization speaks to not only the increasing polarization in the country but also the fact that the Senate looks more and more like the House with each passing year. And it makes the idea of any sort of meaningful bipartisanship any time soon seem like a pie-in-the-sky notion.