matplotlib/matplotlib — pull request #3812 by jbmohler (2014-11-18 16:25:46+00:00)
Title: insert deprecation warning for set_graylevel

Pursuant to comments in https://github.com/matplotlib/matplotlib/pull/3683 , this is a deprecation warning for set_graylevel in the backends.

- it is not used (aside from the inheritance chain calls)
- it has never been used according to git history
- it delivers functionality which is trivial to attain other ways