Here's another graphic I found interesting. House Democrats appear to be much more monolithic and much more left of center than Republicans.
What's interesting is that you (presumably inadvertently) took an image from DW-NOMINATE's voteview blog.
If you are actually interested in which party the people who make that metric view to be further from the ideological center, they have an entire section on political polarization. All you really need is their handy chart however, which I've shown you before:
The mean Republican ideological position is MUCH further to the right than the mean Democratic position. Since you find their charts worth linking to, I presume you accept their methodology as accurate.