Matplotlib plots turn out blank

Question

I am trying to plot circles in matplotlib, but the outcome is always an empty plot.

E.g.

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

fig = plt.figure()
fig.set_size_inches(18.5, 10.5, forward=True)
ax = fig.add_subplot(111, aspect='equal')

x = np.array([17., 29., 41.,  3., 15.])
y = np.array([21., 41., 30., 19., 5.])
r = np.array([22.8035085, 46.04345773, 46.61544808, 16.,  12.16552506])

patches = [mpatches.Circle((xx, yy), rr) for xx, yy, rr in zip(x, y, r)]
collection = mcollections.PatchCollection(patches)
ax.add_collection(collection) 

fig.savefig("test.png")

This produces an empty plot, the same when I try with add_artist. Hopefully someone can point me to where I am going wrong! Thank you

Answer 1

In addition to the other answer provided, you can use ax.autoscale() before saving the plot. that will result in

Answer 2

Your problem here is not the paches haven't been drawed. The reason your plot is blank is that matplotlib didn't auto-adjust the axis according to the range of your patches.

Usually, it will do the auto-adjust jobs with some main plot functions, such as plt.plot(), plt.scatter() .... As far as I'm concerned, it's not designed for drawing geometric figures like triangles, rectangle and circles. This may also explain why this called patches in matplotlib.

So what you need to do is to specify the range of the axis manully by using something like this in your case:

plt.axis([-50, 100, -50, 100])