implement alpha for streamline plots

[kcrisman]

In #10775 we implemented streamline plots, but this ask.sagemath question requests alpha transparency for it too. Currently this raises an error.

Component: graphics

Issue created by migration from https://trac.sagemath.org/ticket/23104

[yuan-zhou]

comment:1

The matplotlib streamplot https://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.streamplot.html currently doesn't support the alpha option. To allow alpha transparency, one would have to re-implement streamline_plot by plotting lines and arrows, instead of using the matplotlib function. As the answer https://ask.sagemath.org/question/37728/transparency-option-in-streamline-plot/?answer=37729#post-id-37729 to the ask.sagemath question suggests, one can use the alpha option in PIL as a workaround.