Plots.jl: Different transparency for markers face and edge

[yakir12]

I want to highlight something in an existing plot by circling it and annotating that. Something like this:

I tried to just add a marker with a transparent face-color with this (under a gr backend):

plot(rand(RGB, 10, 10))
scatter!(9*rand(10) + 1, leg=false, marker = (20, 0.0, stroke(3, 1.0, :black)))

But this doesn't work on the rendered window nor on the saved png.

[mkborregaard]

Yes, it looks like the markerstroke values override the marker values:

scatter(rand(10), markeralpha = 0.2, ms = 20)

scatter(rand(10), markeralpha = 0.2, ms = 20, markerstrokealpha = 1)

[yakir12]

Just a hopefully helpful note to anyone stumbling here, using an RGBA works for the annotations:

annotate!(x, y, text("half transparent red label", RGBA(1,0,0, 0.5)))

[daschw]

The problem (at least) for GR is that in the backend code in order to get a stroke, at first a slightly bigger marker with the :markerstroke color attributes is drawn and then on top the interior with the :markercolor attributes is drawn. Hence, if you reduce :markeralpha, the :markerstrokecolor shines through.
I have not yet found a workaround for this. The only thing I can think of for now is replacing all the markers with Plots.Shapes.

[mkborregaard]

Ugh, that's unfortunate. Replacing markers with Shapes is likely to really slow down plotting.

[mkborregaard]

Of course @yakir12 's original use case can be solved by plotting a circle shape. (e.g. plot(Plots.partialcircle(0, 2π, 100)) if the aspect ratio is 1).