custom colorscale support

[t-bltg]

#2900 / #2493 which landed in 0.19.7 introduced some subtle bugs, where it seems that the colorscale is applied multiple times in the pipeline (resulting in multiple values overlapping on the colorbar).

EDIT: this is related to the inverse transform of custom colorscales.

I still have to come up with an mwe, because my example is complicated.

Currently observed on CairoMakie (other backends un-tested).

• are you running newest version (version from docs) ?

[t-bltg]

Here is a mwe, occurs when using a custom colorscale:

using CairoMakie

main() = begin
  xs = range(0, 2π, length=100)
  ys = range(0, 2π, length=100)
  zs = [sin(x*y) for x ∈ xs, y ∈ ys]

  fig, ax, hm = heatmap(
    xs, ys, zs; colorscale=x -> log10(1 + x)
  )
  Colorbar(fig[2, 1], hm; vertical=false, flipaxis=false)

  display(fig)
  save("colorscale.png", fig; px_per_unit=2)
  return
end

main()

We need IMO some mechanism to register a custom inversible colorscale.

[SimonDanisch]

I think we're re-using parts of the Axis infrastructure, so maybe this helps #3089?

[t-bltg]

Thanks for the reference.

I think the solution implemented in #3095 is generic (works for axis scale, but also for colorscale) ?