Bug report

Bug summary

When using the pgf backend the background cannot be set to transparent.

Code for reproduction

import matplotlib as mpl
mpl.use('pgf')

import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(range(10))
fig.savefig(f"test.png", transparent=True)

Actual outcome

A figure with white background.

Expected outcome

A figure with transparent background. This works when using a different backend (i.e. commenting out mpl.use('pgf')).

Matplotlib version

• Operating system: Windows 10 x64
• Matplotlib version: 2.1.1 (installed with pip)
• Matplotlib backend: pgf
• Python version: 3.6.4