Support mapping strokecolor

[jtrakk]

I know there is AlgebraOfGraphics.density() but I would also like visual(Density) to work. strokecolor doesn't work in the mapping.

let n = 100
(data((;a=rand(n),c=rand('a':'e', n)))
 * mapping(:a, strokecolor=:c)
 * visual(Density, alpha=.1)) |> draw
end


Error: Type Observable{Int64} isn't supported for uniform: color