Provide a 'transparent' color for missing values (isofill/boxfill/etc... plots)

[jypeter]

Would it be possible to plot the missing values with a transparent color? That could probably best be achieved by, when requested, not plotting anything where there is a missing value

This would make it possible to emulate outfill with boxfill (if we have to say goodbye to outfill, #999 (comment)) and would be useful when you want to overlay an isofill/boxfill plot on top of another plot, and want to see some of the data below

The user could specify it the following way

isof = x1.createisofill(None)
isof.missing = None
bf = x1.createboxfill(None)
bf.missing = None

Maybe there is already a way to do this that I don't know of. It's a feature I have needed from time to time, but I did not have time to formally request it

[doutriaux1]

@jypeter I am planning on adding transparency to the colortables, but not for this version. Maybe 2.3 or 3.x

[jypeter]

Will you be using some alpha channel opacity stuff?

What about the not plotting masked cells when the specified missing value is None? This may be naive, but is not the code mostly already in place for that?

[doutriaux1]

@sankhesh I think we pretty much implemented this by now, so I'm reassigning to you so that you can close via git commit.