That is the voteview plot for the current supreme court; which can be contrasted to the same plot for voters in various  states.  The horizontal axis accounts the vast most of voting behavior and is a measure of economic liberal-conservative; i.e. the degree that government should shape markets to the benefit of small v.s. large economic entities.  You can see the distributions for the last congress here, along with where the president fits.  The vertical axis is social issues; higher is more socially conservative.

Update: More here.