[BUG] Surface transparency

[maucejo]

Details

I don't if it's a known issue, but I am struggling with transparency with the gr and pythonplot backends.

I am trying to plot a transparent zx-plane. Here is the MWE:

using Plots

x = LinRange(0., 100., 100)
y = ones(50)

vl = LinRange(0., 1., 50)
v = repeat(vl', length(x), 1)
z = repeat(ones(length(x)), 1, length(vl)).*v

fig = plot()
surface!(y, x, z, c = cgrad([:blue, :blue]), fillalpha = 0.1, colorbar = false)
fig

With gr, when setting fillalpha to a value other than 1, the plane is not plotted. I think it is more related GR itself, since I receive the following warning points not sorted in ascending order. Note that this issue does not appear for a xy-plane.

With pythonplot, fillalpha has no effect at all, while it works well with the pyplot backend. This issue does not appear when I use PythonPlot.jl directly. (It should be noted that the figure displayed with Plots.jl is quite ugly compared to that obtained with PythonPlot.jl, but that is probably another story). This probably means that transparency is not yet implemented for pythonplot backend. Am I right ?

Thank you !

Backends

This bug occurs on ( insert x below )

Backend	yes	no	untested
gr (default)	x
pythonplot	x
pyplot		x
plotlyjs		x
pgfplotsx
unicodeplots			x
inspectdr			x
gaston			x

Versions

Plots.jl version: 1.38.6
Backend version (]st -m <backend(s)>):
PythonPlot v1.0.2
PyPlot v2.11.1
PlotlyJS v0.18.10
GR v0.71.7
Output of versioninfo():
Julia Version 1.8.5
Commit 17cfb8e65e (2023-01-08 06:45 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
CPU: 8 × Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
Threads: 1 on 8 virtual cores
Environment:
JULIA_EDITOR = code
JULIA_NUM_THREADS =

[t-bltg]

That's probably a bug introduced when I ported the pyplot backend to pythonplot.
Thanks for reporting, I'll have a look.

Reduced example:

surface(-10:.1:10, -10:.1:10, (x, y) -> 15sinc(√(x^2 + y^2) / π), c = cgrad([:blue, :blue]), fillalpha = 0.1)

TODO: check that we don't break #4665.