Allow setting alpha (transparency/opacity) for contour_plot

[jasongrout]

The object of this ticket is to allow an optional argument "alpha"
to set the transparency / opacity for contour_plot, like it exists for
other types of plots.

An initial patch enables an 'opacity' argument to the contour_plot function.
It should be renamed 'alpha' for consistency with its name elsewhere.

Doctests need to be added, and a few examples need to be written.

Related tickets and discussions:

• #23104: implement alpha for streamline plots
• #21984: Deprecate the opacity argument for plot3d and use alpha
• Ask Sage question 41457: Fill colors with two contour plots

CC: @slel

Component: graphics

Keywords: alpha, transparency, opacity, plot, contour_plot

Author: Jason Grout

Issue created by migration from https://trac.sagemath.org/ticket/9491

[jasongrout]

Attachment: trac-9491-contour-alpha.patch.gz

[jasongrout]

comment:1

This patch was done in a rush, so there may be bugs in it.

[jasongrout]

comment:2

Comments from David Sanders on sage-support:

Firstly, with this code it seems not to be possible to make a
region_plot without specifying opacity -- it looks like a default
argument is missing somewhere.

[jasongrout]

comment:3

Replying to @jasongrout:

Comments from David Sanders on sage-support:

Firstly, with this code it seems not to be possible to make a
region_plot without specifying opacity -- it looks like a default
argument is missing somewhere.

I think that is correct.

[slel]

Changed keywords from none to alpha, transparency, opacity, plot, contour_plot