This should be fixed together with #632 and #623

I'm planning to look at this tomorrow. Does anyone know why rlocus.py calls its own local _sgrid_func(fig=None, zeta=None, wn=None) instead of grid.sgrid() (which is already imported into the module but is not used). There might be a reason why someone did this...

See these lines:

I'm planning to look at this tomorrow. Does anyone know why rlocus.py calls its own local _sgrid_func(fig=None, zeta=None, wn=None) instead of grid.sgrid()

It is basically a result of two PRs which were started indepently and merged after some time: #617 (comment)

Okay, thanks. So more of an accident than an intention.

Okay, I've run into the same problem from last time I looked at this. The sgrid() function is not compatible with the plotting paradigms we wanted to employ for figures with multiple subplots:

plt.subplot(2, 1, 1)
zgrid()  # this works
plt.subplot(2, 1, 2)
sgrid()  # this won't
plt.show()

and

fig, axes = plt.subplots(2, 1)
zgrid(ax=axes[0, 0])  # this works
sgrid(ax=axes[1, 0])  # this won't
plt.show()

This won't work because sgrid uses a PolarAxes.PolarTransform to create the axis, which you have to do at the point of creating the axis, not after one already exists:

Also, I don't think there's a way to remove an existing axis and then replace it with the desired polar axis (could be wrong on this point).

Welcome comments on this but I think there is one solution...

The only solution I can think of right now is to require a slightly different set of arguments when adding an sgrid to an existing figure. Based on this example script from the documentation, I think this might work but I haven't tried implementing it yet:

fig = plt.figure()
sgrid(fig=fig, position=(2, 1, 1))  # this should be do-able
zgrid(fig=fig, position=(2, 1, 2))  # add the same functionality to zgrid
plt.show()

From the documentation, it sounds like the subplots method was intended as a convenience method for making plots of the same type only. It does have two keyword arguments for creating different types of axes and grids, subplot_kw and gridspec_kw but I'm not sure you can specify different types for each subplot (I will check this).

Question. I'm in the process of replacing _sgrid_func with sgrid and I noticed that _sgrid_func uses two other functions in rlocus.py:

Shall I delete these functions (they were only used by _sgrid_func) or should these features be carried over?

fig, axes = plt.subplots(2, 1)
zgrid(ax=axes[0, 0])  # this works
sgrid(ax=axes[1, 0])  # this won't
plt.show()

fig = plt.figure()
sgrid(fig=fig, position=(2, 1, 1))  # this should be do-able
zgrid(fig=fig, position=(2, 1, 2))  # add the same functionality to zgrid
plt.show()

You can always get the parent figure of a given axis by ax.get_figure() and the position by ax.get_subplotspec(). So whatever you can achieve by the second form should also be possible with the first form.

This won't work because sgrid uses a PolarAxes.PolarTransform to create the axis, which you have to do at the point of creating the axis, not after one already exists:

python-control/control/grid.py

Lines 88 to 93 in 6b7a9c7

	grid_helper = GridHelperCurveLinear(
	tr, extreme_finder=extreme_finder, grid_locator1=grid_locator1,
	tick_formatter1=tick_formatter1)
	fig = plt.gcf()
	ax = SubplotHost(fig, 1, 1, 1, grid_helper=grid_helper)

Also, I don't think there's a way to remove an existing axis and then replace it with the desired polar axis (could be wrong on this point).

The MPL Curvilinear grid demo where this was taken from has changed a little bit since then. Maybe this needs an update too.

You can always get the parent figure of a given axis by ax.get_figure() and the position by ax.get_subplotspec(). So whatever you can achieve by the second form should also be possible with the first form.

The problem is not getting access to the figure, it is that the axes has to be created in a certain way by sgrid. sgrid cannot work with an axis that is already created so it has to be a blank figure (before the axis was created). Unless someone knows a way to remove an axis already created so it can be replaced.

I am not sure about this. Maybe you can shuffle around existing axes objects together with a newly created parasite axis using the right combination of delaxes(ax), fig.add_subplot(ax), etc.

Thanks, delaxes(ax) looks promising. I will check out later this week.