PR Summary

As pointed out by @anntzer constrained layout gets slow for many subplots. Some slowdown is to be expected, but there were numerous redundant constraints being added, so this now has watchdogs to make sure only one constraint is added. Also some (probably insignificant) optimizations to reduce the number of calculations.

"closes" #10655

• subplots(4, 4) took 4s now 1.8s
• subplots(5, 5) took 12s, now 3.3s
• subplots(6, 6) took 30s, now 6.3s

Still not quite linear in N, but a lot better than N*2 which it was before.

import matplotlib.pyplot as plt
import timeit

start = timeit.default_timer()

fig, ax = plt.subplots(6, 6, constrained_layout=False)
fig.canvas.draw()
stop = timeit.default_timer()
print(stop-start)

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