Bug report

When saving a png of a polar plot with the r-origin set "by hand," saving the result as a png plot doesn't work as expected. Saving it as PDF works as expected. Working interactively and saving it with the UI as a png works as expected.

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.gridspec import GridSpec

nbins = 50
data = np.random.randint(0, 42, nbins)

fig = plt.figure()

ax_g = fig.add_subplot(polar=True, facecolor="none")
theta = np.array(
    [np.pi/2-i*2*np.pi/len(data) for i in range(len(data))])
lines = ax_g.plot(theta, data)

ax_g.set_rorigin(-78)  # Replacing this with ax_g.set_rmin works as expected, so the problem comes from this line.

fig.savefig("bug.png")
fig.savefig("bug.pdf")

Actual outcome

Expected outcome

bug.pdf

Matplotlib version

• Operating system: Ubuntu
• Matplotlib version: 3.2.1
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg (tried with Agg, and other backends)
• Python version: 3.8.2
• Jupyter version (if applicable): NA
• Other libraries:  NA