`xflip=true` reverses colormaps in saved PDFs with GR backend

[brenhinkeller]

Details

compare

A = repeat(1:10,1,10) # Test image with an obvious pattern
h = heatmap(A)
savefig(h, "test.pdf") # Works as normal

with

A = repeat(1:10,1,10) # Test image with an obvious pattern
hr = heatmap(A, xflip=true)
savefig(hr, "test_xflip.pdf") # Colorscale is now reversed

This does not appear to affect other file types (PNG and SVG tested normal, only PDF is reversed so far), and impact both heatmaps and at least some other images (I originally noticed this in an externally-scaled image annotated using Plots here).

Backends

This bug occurs on ( insert x below )

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

Versions

Plots.jl version: [91a5bcdd] Plots v1.40.17
Backend version (]st -m <backend(s)>): [28b8d3ca] GR v0.73.17
Output of versioninfo():

Julia Version 1.11.6
Commit 9615af0f269 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 8 default, 0 interactive, 4 GC (on 8 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 8

[brenhinkeller]

I haven't tracked exactly what the source of this is, but it may be related to a change in the GR backend itself, since downgrading GR from v0.73.17 to v0.73.16 seems to restore the original color scale