For what it's worth, I thought the gray background bars were far too light - it took me a while to find them. I also couldn't intuit what the branching lines were all about.
Also, Chrome had some difficulty with typesetting, though it's possible that's a localized issue.
*edit: I figured out why the boxes seem so light - it's because I view my screen at a slight angle. From a straight angle they show up fine.
Sorry about that, I should have clarified that each Senator could be in different groups, hence the branching. I'll add a little darkness to the background bars.
What kind of typesetting issues did you have, and with which version of Chrome?
The visualization is quite nice, and it would be useful to link to or include some kind of short description about the algorithm and visualization - to give a different example, it looks like the groups are scored numerically but the senators are simply ranked, so I wonder if the algorithm found more than two groups per congress.
I have absolutely no idea why it renders this way on your Chrome, maybe an issue with TypeKit?
About the visualization, it was done semi-manually with a tool I've developed to heuristically minimize the crossings, but their is no way to obtain this kind of layout purely algorithmically (I'm pretty sure it's NP-hard).
it looks like the groups are scored numerically but the senators are simply ranked
Not quite sure what you mean. The groups were positioned in order to have the one with most Republicans on one side and the one with most Democrats on the other one. There were consistently two groups for those sessions, this I can assure you. The order of the senators inside the groups is arbitrary, in order to attempt to minimize the number of crossings and provide a readable output.
Ah, I assumed each senator was placed according to some measure of extremism, though in hindsight that alone should have produced crossings.
To support this interpretation, I noticed that the groups are not aligned - see for example George Allen's swerve in the 108th Congress. I suppose I was expecting it too, from having seen some visualization that tried and failed to find a tea party cluster.
Also, Chrome had some difficulty with typesetting, though it's possible that's a localized issue.
*edit: I figured out why the boxes seem so light - it's because I view my screen at a slight angle. From a straight angle they show up fine.