Description

josujugo

opened on May 22, 2019

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Recently, the Nyquist plot adds arrows to show the directions in the diagram. However, those arrows have always the same size and the same position. Working with my students, sometimes the arrows make difficult the correct interpretation of the Nyquist plots.

New options for the arrow size and position (not always the first and the last frequencies) could help to solve the issue .

Reacted by geekonloose and Pablo

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1. murrayrm

   added

   enhancement

   on May 22, 2019
2. geekonloose commented on Mar 3, 2020

   

   geekonloose

   on Mar 3, 2020

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   Yes, I am also experiencing this problem. Is there any hack to reduce the size of arrows?
   For example figure below,
   
3. murrayrm commented on Mar 3, 2020

   

   murrayrm

   on Mar 3, 2020

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   The problem seems to be that the arrow dimensions are based on the axis units rather than an "absolute" size (e.g., pixels). It should be relatively easy to add an option to nyquist_plot to set the arrow size. Better would be to set the arrrow size so that it isn't in axis units.

   If someone wants to generate a PR for that, it would be useful. I won't have time to get to it for a while.

   Quick hack: you might be able to get the arrow object with matplotlib and reset its size.
4. murrayrm commented on Mar 18, 2020

   

   murrayrm

   on Mar 18, 2020

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   PR #379 provides a quick fix that allows the arrowhead size to be set manually.

   A better algorithm that computes arrow head size automatically is still needed at some point. Arrow position is also not yet done in a good way.
5. kobemertens commented on Aug 19, 2020

   

   kobemertens

   on Aug 19, 2020

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   I'm not too familiar with the issue but the matplotlib documentation suggests using an annotation with an empty string as text like this:

   >>> ax.annotate("", xy=(0.5, 0.5), xytext=(0, 0),
   ...             arrowprops=dict(arrowstyle="->"))

   Maybe this is worth looking into?
6. bnavigator commented on Aug 19, 2020

   

   bnavigator

   on Aug 19, 2020

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   Hi @SchrodingersCat00, that sounds like a good solution, especially since the current code also just adds a dummy straight line for the arrow from the edges of the actual curve.

   python-control/control/freqplot.py

   Lines 534 to 555 in f3e5035

   	# Compute the primary curve
   	x = np.multiply(mag, np.cos(phase))
   	y = np.multiply(mag, np.sin(phase))
   	if plot:
   	# Plot the primary curve and mirror image
   	p = plt.plot(x, y, '-', color=color, *args, **kwargs)
   	c = p[0].get_color()
   	ax = plt.gca()
   	# Plot arrow to indicate Nyquist encirclement orientation
   	ax.arrow(x[0], y[0], (x[1]-x[0])/2, (y[1]-y[0])/2, fc=c, ec=c,
   	head_width=arrowhead_width,
   	head_length=arrowhead_length)
   	plt.plot(x, -y, '-', color=c, *args, **kwargs)
   	ax.arrow(
   	x[-1], -y[-1], (x[-1]-x[-2])/2, (y[-1]-y[-2])/2,
   	fc=c, ec=c, head_width=arrowhead_width,
   	head_length=arrowhead_length)
   	# Mark the -1 point
   	plt.plot([-1], [0], 'r+')

   Do you want to submit a PR for that?
7. murrayrm commented on Feb 11, 2021

   

   murrayrm

   on Feb 11, 2021

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   PR #534 fixes this issue.
8. murrayrm

   linked a pull request that will close this issueNyquist plot improvements #534on Feb 11, 2021
9. murrayrm

   closed this as completedin #534on Feb 12, 2021

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