[BUG] Colorbar colors don't match actual colors in `contourf` plot

[david-macmahon]

Details

Using specified levels with the contourf function, I get colorbar colors that don’t match the plot. This happens when using the GR backend. The Plotly/PlotlyJS backends claim not to support arbitrary contour levels, so this can't be tested with that backend. Here is an example:

julia> z = 50 .- ((-5:5).^2 .+ ((-5:5).^2)');
julia> contourf(z, levels=[0,20,30,45,50], fillcolor=:lightrainbow, clabels=true)

This produces this plot where the 20 to 30 band in the plot doesn’t have the same color as shown on the colorbar for that range:

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: Plots v1.40.4 Backend version (]st -m <backend(s)>): GR v0.73.6 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: 8 × Apple M2
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
Threads: 4 default, 0 interactive, 2 GC (on 4 virtual cores)
Environment:
  JULIA_EDITOR = code
  DYLD_FALLBACK_LIBRARY_PATH = [super long path with lots of Julia artifact directories elided]