In using matplotlib (after PR#9356) on PyPy, I ran into an issue that theTkAGGSubplotTool python object was being collected by the PyPy garbage collector (GC). It turns out that theSubplotTool holds circular references, and these are resolved in the CPython GC much much later than in PyPy. I also noticed that theSubPlotTool onTkAGG is not a modal window, I can open many at the same time.

PR Summary

This pull request solves both problems: holds a reference to theSubplotTool and makes the dialog into a modal dialog.

Here is code that provides a basis for a manual test, I am not sure how to test the modality of a dialog box via a unittest:

import matplotlibmatplotlib.use('TkAgg')from matplotlib import pylabfig = pylab.plot(range(10))pylab.show()# press the subplot button in the toolbar# try to press the button again, the SubplotTool dialog should hold focus# make sure the SubplotTool is functioning, that the sliders affect the main window

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way