Making figure transparent with colored background

Question

I have a bit of a situation. What I need is a plot with a black background with several white circles drawn on top of that black background.

I managed to do this usi

Answer 1

This might not be the answer you are looking for but it gives the picture you wanted! I think you want to fill the areas outside the circle!(s) with black and leave the background transparent, rather than the other way around. It's trivial to calculate the boundaries of a single circle and use fill_between. Doing it for multiple circles might be trickier!

import numpy as np
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(1, 1, 1, aspect = "equal", )

# A Circle
xy=(1,1); r=3

# more points is smoother
xdata=np.linspace(-5,5,1001)
ydata=np.linspace(-5,5,1001)

# circle edges (top and bottom)
c1=np.sqrt((xy[0]**2-xdata**2)+r**2)+xy[1]
c2=-np.sqrt((xy[0]**2-xdata**2)+r**2)+xy[1]

c1=np.where(np.isnan(c1),xy[0],c1)
c2=np.where(np.isnan(c2),xy[0],c2)

ax.fill_between(xdata,5,c1,color='black')
ax.fill_between(xdata,-5,c2,color='black')

plt.xlim(-5, 5)
plt.ylim(-5, 5)

fig.savefig("test.png", dpi = 300, transparent=True)

Transparent circle with center 1,1 and radius 3