Remove broken printing_in_wx.py example.

It shows a preview, but writes blank files.  The wx printer
support has been deprecated for more than 2 years.
Merge pull request #1756 from efiring/del_wx_print_example

Remove broken printing_in_wx.py example.
Re-write stacked step histogram
Fix bug in stacked bar histogram. Also, calling hist with `log=True` now
behaves correctly in all cases.