BUG: scatter_matrix problem, ranges of diagonal and off-diagonal plots are different

In [this](https://github.com/pydata/pandas/issues/5497) issue I signaled that the x_lim and y_lim in scatter_matrix plots are different. In particular x/y_lim of diagonal hists and x/y_lim of scatter plots are different.  

For instance, this causes peaks nonalignments in the 1-margin and 2-margin of the plotted multivariate pdf. See pictures in the issue.


This is a possible fix.