Make a single legend entry for a contourf plot by stacking proxy artists together. Proposed solution for https://stackoverflow.com/questions/77842076

contourf_legend.py

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import numpy as np

x, y = np.meshgrid(np.arange(10),np.arange(10))
z = np.sqrt(x**2 + y**2)

fig, ax = plt.subplots()

cs = ax.contourf(x,y,z,levels=[2,3,4,6])
colors = cs.get_facecolors()

proxies = []
for size, color in enumerate(colors, 1):
    proxies.append(
        mlines.Line2D([], [], color=color, marker='o', ls='none', markersize=size))
    
proxies.reverse()  # Put smallest marker on top

ax.legend(handles=[tuple(proxies)], labels=['My Contour'], borderpad=1.5, markerscale=10)

plt.show()

See the Legend Guide for more ideas and explanation.

Output: