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josujugo opened this issue May 22, 2019 · 6 comments · Fixed by #534
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arrows in nyquist plot #303

josujugo opened this issue May 22, 2019 · 6 comments · Fixed by #534
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@josujugo
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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 .
@geekonloose
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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,
Figure_1

@murrayrm
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murrayrm commented Mar 3, 2020

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.

@murrayrm murrayrm mentioned this issue Mar 4, 2020
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@murrayrm
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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.

@kobemertens
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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?

@bnavigator
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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?

@murrayrm
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PR #534 fixes this issue.

@murrayrm murrayrm linked a pull request Feb 11, 2021 that will close this issue
Nyquist plot improvements #534
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Nyquist plot improvements murrayrm/python-control
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@murrayrm @bnavigator @geekonloose @kobemertens @josujugo