Question

I would like to plot a circle on an auto-scaled pyplot-generated graphic. When I run

ax.get_aspect()

hoping for a value with which I could manipulate the axes of a ellipse, pyplot returns:

auto

which is less than useful. What methods would you suggest for plotting a circle on a pyplot plot with unequal axes?

Answer 1

It really does depend what you want it for.

The problem with defining a circle in data coordinates when aspect ratio is auto, is that you will be able to resize the figure (or its window), and the data scales will stretch nicely. Unfortunately, this would also mean that your circle is no longer a circle, but an ellipse.

There are several ways of addressing this. Firstly, and most simply, you could fix your aspect ratio and then put a circle on the plot in data coordinates:

import matplotlib.pyplot as plt
import numpy as np


fig = plt.figure()
ax = plt.axes()

ax.set_aspect(1)
theta = np.linspace(-np.pi, np.pi, 200)
plt.plot(np.sin(theta), np.cos(theta))

plt.show()

With this, you will be able to zoom and pan around as per usual, but the shape will always be a circle.

If you just want to put a circle on a figure, independent of the data coordinates, such that panning and zooming of an axes did not effect the position and zoom on the circle, then you could do something like:

import matplotlib.patches as mpatches
import matplotlib.pyplot as plt
import numpy as np


fig = plt.figure()
ax = plt.axes()
patch = mpatches.Circle((325, 245), 180, alpha=0.5, transform=None)
fig.artists.append(patch)


plt.show()

This is fairly advanced mpl, but even so, I think it is fairly readable.

HTH,