When using matplotlib in the Jupyter Notebook with the notebook (or nbagg) backend, multiple re-executions of of a cell (containing a plot) cause some problems.

1. When using the procedural API: after the second execution of the same cell no figure is displayed (until the next close() call).
2. When using the OO API: after each execution, the old fig and ax objects are overwritten, a new figure is created and the RAM is filled with invisible figures. Even plt.close('all') does not free any RAM.

Re-executing a cell to tweak or add elements to a plot is a very common use case during interactive sessions.

This issue was first mentioned in #4456.