Bug report

Bug summary

When pcolorfast uses pcolorimage mode (i.e., for rectangular meshes with unequal spacing), an alpha keyword gets silently dropped even though there appears to be code to handle that in the definition of pcolorfast:

            elif style == "pcolorimage":
                im = mimage.PcolorImage(self, x, y, C,
                                        cmap=cmap,
                                        norm=norm,
                                        alpha=alpha,  # <- here
                                        **kwargs)

Code for reproduction

plt.gca().pcolorfast(
    [4, 8, 10], [7, 8, 10, 11], np.array([[0, 1], [2, 3], [4, 5]]),
    alpha=.5, cmap="viridis")

Actual outcome

The alpha channel is not applied. Compare e.g. with

plt.gca().pcolorfast(
    (0, 1), (2, 3), np.array([[0, 1], [2, 3], [4, 5]]),
    alpha=.5, cmap="viridis")

which uses AxesImage and correctly applies alpha.

Expected outcome

per above

Matplotlib version

• Operating system: linux
• Matplotlib version: 3.0 and master
• Matplotlib backend (print(matplotlib.get_backend())): qt5agg
• Python version: 3.7
• Jupyter version (if applicable): n/a
• Other libraries: n/a