[WIP] Customizable colorrange handling

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## [Unreleased]
 - Added supported markers hint to unsupported marker warn message.
-
+- `colorrange` can now accept any `Function` which returns a 2-tuple or length-2 vector, as well as an extensible interface for users to integrate their own colorrange objects. [#3688](https://github.com/MakieOrg/Makie.jl/pull/3688)
 - Remove StableHashTraits in favor of calculating hashes directly with CRC32c [#3667](https://github.com/MakieOrg/Makie.jl/pull/3667).
 
 ## [0.20.8] - 2024-02-22

src/colorsampler.jl

@@ -244,6 +244,28 @@ colormapping_type(@nospecialize(colormap)) = continuous
 colormapping_type(::PlotUtils.CategoricalColorGradient) = banded
 colormapping_type(::Categorical) = categorical
 
+"""
+ evaluate_colorrange(colorrange, color)::Vec2{Float64}
+
+This function must return a `Vec2{Float64}` of the colorrange as `(low, high)`.
+Add additional dispatches to this function for your own custom colorrange objects!
+
+The catch-all definition for this function is to convert any provided `colorrange` into 
+a `Vec2{Float64}`. By default, two more dispatches are defined for `Makie.automatic` 
+and for `Function`s (but not all callables, since that's not possible to define on).
+
+You can define your method like so:
+
+```julia
+Makie.evaluate_colorrange(colorrange::MyCustomType, color) = ...
+```
+
+and `color` is always an array of numbers. Be warned that the vector may have only one element.
+"""
+evaluate_colorrange(colorrange, color) = Vec2{Float64}(colorrange)
+evaluate_colorrange(colorrange::Automatic, color) = Vec2{Float64}(Makie.distinct_extrema_nan(color))
+evaluate_colorrange(colorrange::Function, color) = Vec2{Float64}(colorrange(color))
+
 
 function _colormapping(
  color_tight::Observable{V},
@@ -292,7 +314,7 @@ function _colormapping(
  end
 
  colorrange = lift(color_tight, colorrange; ignore_equal_values=true) do color, crange
- return crange isa Automatic ? Vec2{Float64}(distinct_extrema_nan(color)) : Vec2{Float64}(crange)
+ return evaluate_colorrange(crange, color)
  end
 
  colorrange_scaled = lift(colorrange, colorscale; ignore_equal_values=true) do range, scale