The two tightly coiled clusters shows just how weak the relationships are between the two parties.
The closer the nodes are to the center, the more bipartisan those Senators are. In the current Congress, Sens. Murkowski and Collins really stand out as by far the most bipartisan senators and important linchpins in bridging the gap between both parties. (this bar graph of partisanship shows that: http://i.imgur.com/gYCMaxo.png)
Also, it’s clear that in the current Senate, partisanship is a lot more pronounced among Democrats, likely because they hold the majority in the Senate. (ie. they have a lot less incentive to compromise)
While there were ups and downs, it’s clear that there’s a general trend of the Senate becoming a lot more divided over the last 25 years. Comparing 1989 to today is striking — opinions were a lot more intertwined back then (this static image really emphasize it: http://i.imgur.com/4kDokyz.jpg).