Overlaying two plots using pcolor

Question

I want to plot two numpy arrays Z1 and Z2 in the same figure, Z2 on top of Z1. The array Z2 contains only 0's and 1's, and I want 0's to be fully transparent (alpha = 0) and 1's transparent with some alpha > 0.

Here's the code and the resulting image:

import numpy as np
import matplotlib.pyplot as plt

N = 10
x = np.arange(0, N)
y = np.arange(0, N)
Z1 = np.random.rand(N,N)
Z2 = np.ones((N, N))
Z2[0:N//2, 0:N] = 0

X, Y = np.meshgrid(x, y)
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(8, 6))
plt.pcolormesh(X, Y, Z1, cmap=plt.cm.Blues)
plt.colorbar()
plt.pcolormesh(X, Y, Z2, cmap=plt.cm.Reds_r, alpha=0.3)
ax.set_xlabel(r'$x$', fontsize=22)
ax.set_ylabel(r'$y$', fontsize=22)
plt.show()

There are two problems:

1. Appearance of the unwanted grid lines

2. The 0's of Z2 are not fully transparent as needed

To get rid of the grid lines we can use imshow instead of pcolor, but I really want to use the values of x and y.

Answer 1

The easiest option to get some of the pixels transparent is to not draw them at all. This would be done by setting them to NaN (not a number).

import numpy as np
import matplotlib.pyplot as plt

N = 10
x = np.arange(0, N)
y = np.arange(0, N)
Z1 = np.random.rand(N,N)
Z2 = np.ones((N, N))
Z2[0:N//2, 0:N] = np.nan

X, Y = np.meshgrid(x, y)
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(8, 6))
plt.pcolormesh(X, Y, Z1, cmap=plt.cm.Blues)
plt.colorbar()
plt.pcolormesh(X, Y, Z2, cmap=plt.cm.Reds, vmin=0,vmax=1,alpha=0.3)
ax.set_xlabel(r'$x$', fontsize=22)
ax.set_ylabel(r'$y$', fontsize=22)
plt.show()