matplotlib pyplot pcolor savefig colorbar transparency

Question

I am trying to export a pcolor figure with a colorbar. The cmap of the colorbar has a transparent color. The exported figure has transparent colors in the axes but not in the colorbar. How can I fix this?

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

x = np.random.random((10, 10))
colors = [(0,0,0,0), (0,0,0,1)]
cm = LinearSegmentedColormap.from_list('custom', colors, N=256, gamma=0)
plt.pcolor(x,cmap=cm)
plt.colorbar()
plt.savefig('figure.pdf',transparent=True)

I put the image against a grey background to check. As can be seen, the cmap in the axes is transparent while the one in the colorbar is not.

Answer 1

While the colorbar resides inside an axes, it has an additional background patch associated with it. This is white by default and will not be taken into account when transparent=True is used inside of savefig.

A solution is hence to remove the facecolor of this patch manually,

cb.patch.set_facecolor("none")

A complete example, which shows this without actually saving the figure

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

x = np.random.random((10, 10))
colors = [(1,1,1,0), (0,0,0,1)]
cm = LinearSegmentedColormap.from_list('custom', colors, N=256, gamma=0)

fig, ax = plt.subplots(facecolor="grey")

im = ax.pcolor(x,cmap=cm)
cb = fig.colorbar(im, drawedges=False)

ax.set_facecolor("none")
cb.patch.set_facecolor("none")

plt.show()