matplotlib gradient overlays - gradient resolution is different from image resolution

mpl_gradient_overlay.py

import matplotlib.pyplot as plt
import numpy as np
from cmap import Colormap


h, w = 512, 868
image = np.random.random(size=(h, w))
defocus = np.array([[0, 1],
                    [1, 0]])

fig, ax = plt.subplots()
ax.imshow(image, cmap='gray')
cm = Colormap('tol:BuRd').to_mpl()
ax.imshow(defocus, extent=(-0.5, w-0.5, h-0.5, -0.5), alpha=0.3, interpolation='spline36', cmap=cm)
plt.show()