scatter_matrix: coordinate misalignment among diagonal and off-diagonal plots
I make extensive use of `scatter_matrix` and I find misalignments in the coordinates of the scatter plots (off-diagonal) and the histograms (on-diagonal) which seem undesirable to me.

Consider the picture ![example](https://f.cloud.github.com/assets/2300692/1522061/389753b0-4ba1-11e3-97f0-31caab31fcb7.png)
). 

In this case, there is a clear misalignment (of about .02) in the x-coordinate between the subplot in position 1,2 and the subplot in position 2,2. 
In other words, the peeks in subplots 1,2 and 2,2 should occur at the same x location 
(at least, IMHO, this would be more meaningful behavior).
