fix ax processing bug in {nyquist,nichols,describing_function}_plot by murrayrm · Pull Request #1145

fix ax processing bug in {nyquist,nichols,describing_function}_plot by murrayrm · Pull Request #1145 · python-control/python-control

This PR fixes #1144 (also pointed out in #1143) where the ax keyword was being ignored in the nyquist_plot command.

Turns out that there were several related bugs:

The Nichols grid was not being plotted in the proper location when ax was passed to nichols_plot.
The describing function curve was not being plotted in the proper location when ax was passed to describing_funciton_plot.

This PR fixes those bugs as well, and includes an updated unit test that picks up all three previous errors.