[BUG] GR backend, PDF output, transparent background: opacity of grid lines differs

[stepanzh]

Details

GR backend does not respect opacity of x-grid lines when saving to PDF with background=:transparent (while saving to PNG is OK). The original issue comes from stackoverflow, https://stackoverflow.com/questions/79007982/dark-vertical-lines-in-a-transparent-plot.

The issue seems independent of PDF viewer (tested in Preview, Skim, Acrobat Reader, Mozilla). The screenshots below are from Preview (a default PDF viewer on MacOS).

Example 1 - Single plot

Note that opacity differs for x- and y-grid lines.

using Plots
gr()

grid = (:both, :red, :solid, 5, 0.25)

plot(; background=:transparent, grid=grid)
savefig("first.pdf")

Example 2 - Multiple plots

Surprisingly, rendering of grid lines is normal for subplots, except first.

using Plots
gr()

grid = (:both, :red, :solid, 5, 0.25)

plot(; layout=(2, 2))
plot!(; background=:transparent, grid=grid)
plot!(; background=:transparent, grid=grid)
plot!(; background=:transparent, grid=grid)
plot!(; background=:transparent, grid=grid)
savefig("both.pdf")

Backends

This bug occurs on ( insert x below )

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

Versions

Plots.jl version: 1.40.8
Backend version (]st -m <backend(s)>): 0.73.7
Output of versioninfo():

Julia Version 1.10.4
Commit 48d4fd48430 (2024-06-04 10:41 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M2 Pro
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 6 virtual cores)