Originally posted by Silent Master
Number of each side in office doesn't tell us anything about how far right/left their positions are. if you want to see what direction they're moving in, you have to compare the left/right's current positions with the positions they held in the past.
The X axis measures how far from the political centre the numbers are. The numbers themselves aren't really the issue as they'll fluctuate depending on how many of each party are in office at any given time. The graphs show that the Dems have shifted less to the liberal side than the repubs have to the conservative end.