Inconsistent behavior for savefig.facecolor + savefig.transparent depending on output format #9921

Bug report

Bug summary

Setting both savefig.facecolor and savefig.transparent behaves differently depending on whether the image is saved as jpeg or png.

Code for reproduction

modified from test_image.test_jpeg_alpha

from pylab import *
rcdefaults()

plt.figure(figsize=(1, 1), dpi=300)
# Create an image that is all black, with a gradient from 0-1 in
# the alpha channel from left to right.
im = np.zeros((300, 300, 4), dtype=float)
im[..., 3] = np.linspace(0.0, 1.0, 300)

plt.figimage(im)

with rc_context({'savefig.facecolor': 'red', 'savefig.transparent': True}):
    plt.savefig("/tmp/test.jpg")
    plt.savefig("/tmp/test.png")

Actual outcome

test.png

test.jpg

Expected outcome

Some consistent behavior across formats (the fact that jpeg cannot represent transparency should not prevent either both files to have a red background or both files not to have a red background).

I plan to deprecate savefig.facecolor/savefig.edgcolor/savefig.transparent anyways (see #9080, #9698); mostly just noting the issue in the meantime.

Matplotlib version

• Operating system:
• Matplotlib version: master and 2.1
• Matplotlib backend (print(matplotlib.get_backend())):
• Python version:
• Jupyter version (if applicable):
• Other libraries: