Two 'sgrid' functions when we only need one?

As pointed out in PR #617, there are now two sgrid functions in the package: grid.py:sgrid and rlocus.py:_sgrid_func. It looks like _sgrid_func was removed in PR #193 (discrete omega-damping plot and tweaks by Sup3rGeo), but was re-introduced in PR #209 (sisotool and dynamic root locus zoom by icam0).

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:

python-control/control/rlocus.py

Lines 234 to 239 in 6b7a9c7

    
           # Draw the grid 
        
           if grid: 
        
               if isdtime(sys, strict=True): 
        
                   zgrid(ax=ax) 
        
               else: 
        
                   _sgrid_func(fig=fig if sisotool else None)

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:

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).

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:

python-control/control/rlocus.py

Lines 712 to 718 in 6b7a9c7

    
           def _default_zetas(xlim, ylim): 
        
               """Return default list of damping coefficients 
        
               This function computes a list of damping coefficients based on the limits 
        
               of the graph.  A set of 4 damping coefficients are computed for the x-axis 
        
               and a set of three damping coefficients are computed for the y-axis 
        
               (corresponding to the normal 4:3 plot aspect ratio in `matplotlib`?).

python-control/control/rlocus.py

Lines 750 to 754 in 6b7a9c7

    
           def _default_wn(xloc, yloc, max_lines=7): 
        
               """Return default wn for root locus plot 
        
               This function computes a list of natural frequencies based on the grid 
        
               parameters of the graph.

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

Hi bill, I’ve never seen them used anywhere else, and as private functions I think you can delete them if they don’t show up in anywhere else in the library.

On Sun, Aug 15, 2021 at 2:54 PM Bill Tubbs ***@***.***> wrote: 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: https://github.com/python-control/python-control/blob/6b7a9c78f16ab04e009be7155308294080d8a5d1/control/rlocus.py#L712-L718 https://github.com/python-control/python-control/blob/6b7a9c78f16ab04e009be7155308294080d8a5d1/control/rlocus.py#L750-L754 Shall I delete these functions (they were only used by _sgrid_func) or should these features be carried over? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#634 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AN74SSONX6UL556R6PH4OGDT5AZPNANCNFSM46YOSZAQ> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&utm_campaign=notification-email> .

-- Sawyer Fuller Assistant Professor of Mechanical Engineering Adjunct, Paul G Allen School of Computer Science University of Washington http://faculty.washington.edu/~minster/ (Typed with my thumbs)

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.

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Two 'sgrid' functions when we only need one? #634

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	# Draw the grid
	if grid:
	if isdtime(sys, strict=True):
	zgrid(ax=ax)
	else:
	_sgrid_func(fig=fig if sisotool else None)

	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)

	def _default_zetas(xlim, ylim):
	"""Return default list of damping coefficients

	This function computes a list of damping coefficients based on the limits
	of the graph. A set of 4 damping coefficients are computed for the x-axis
	and a set of three damping coefficients are computed for the y-axis
	(corresponding to the normal 4:3 plot aspect ratio in `matplotlib`?).

	def _default_wn(xloc, yloc, max_lines=7):
	"""Return default wn for root locus plot

	This function computes a list of natural frequencies based on the grid
	parameters of the graph.

Two 'sgrid' functions when we only need one? #634

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bnavigator commented on Jun 16, 2021

billtubbs commented on Aug 15, 2021

bnavigator commented on Aug 15, 2021

billtubbs commented on Aug 15, 2021

billtubbs commented on Aug 15, 2021

billtubbs commented on Aug 15, 2021

billtubbs commented on Aug 15, 2021

sawyerbfuller commented on Aug 16, 2021

bnavigator commented on Aug 16, 2021

bnavigator commented on Aug 16, 2021

billtubbs commented on Aug 16, 2021

bnavigator commented on Aug 16, 2021

billtubbs commented on Aug 16, 2021

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