It's very good, but suffers from one basic presentational error: You've placed Republicans on the left and Democrats on the right. While I know your algorithm doesn't attempt to plot people on a particular spectrum, it so happens that stereotypically partisan conservative or liberal senators tend to be positioned at the outer margins anyway. since liberals are traditionally thought of as left-leaning and conservatives as right-leaning [1], I would suggest your diagram needs to be flipped horizontally.

1. http://en.wikipedia.org/wiki/Left%E2%80%93right_politics#His...