reduce number of draws to increase speed

[ianhi]

On my laptop this took the dynamic example playback from:

18.7 s ± 1.15 s per loop (mean ± std. dev. of 7 runs, 1 loop each)

to
8.33 s ± 120 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

This works because writer.grab_frame calls savefig which ultimately calls draw