`density` with gradient color cannot be added to `Legend`

[fatteneder]

MWE

using GLMakie
GLMakie.activate!()

f = Figure()
ax = Axis(f[1,1])

density!(ax, randn(200) .- 2sin(2/3*pi),
         color = :x, colormap = :thermal, colorrange = (-5, 5),
         strokewidth = 1, strokecolor = :black, label="density")

Legend(f[1,2], ax)

f

throws

julia> include("new_mwe.jl")
ERROR: LoadError: Unknown color: x

The above works if any other <:Colorant is given and it then plots a little square with that color as the marker in the Legend.

Should we plot a squeezed colorbar as the legend marker in this case?

Btw: I think the same question also applies to a heatmap. However, there isn't an overload for of legendelements for it yet.

[fatteneder]

A squeezed colorbar is perhaps not ideal because the markers in the legend are usually relatively small compared to a real colorbar.
Also: If you have two density plots and you put each one into a Legend, then having the same squeezed colorbar as a marker would not help you in telling them apart in the plot.

A simpler solution, which is also employed by gnuplot, is to put only the label and no marker in the legend.

[jkrumbiegel]

That's why I never implemented automatic legend items for things with continuous color maps. It's just not obvious to me what the behavior should be like.

[fatteneder]

Imitating gnuplots behavior is not an option?
The only reason that would be useful is to have a target you can click for the hide and show functionality of #2276, assuming it gets merged.

[jkrumbiegel]

Do you have some visual examples? Never used gnuplot

[fatteneder]

Here is a gnuplot demo which you can reproduce with

$ gnuplot
gnuplot> splot cos(x) * cos(y) w pm3d, sin(x) * sin(y) w p

Explanation:
Plotting with palette-mapped 3d (that's what w pm3d says) adds a colorbar as well as a legend entry with cos(x) * cos(y), but with no marker (top right).
Plotting with points (that's what w p says) does not add a colorbar, but adds a legend entry with a little purple cross marker.

[jkrumbiegel]

Ok but I thought you'd show me a legend with a marker for a continuous color surface. This legend doesn't have that, there's just a separate colorbar

[fatteneder]

That's what I wrote earlier:

A simpler solution, which is also employed by gnuplot, is to put only the label and no marker in the legend.

[fatteneder]

Coming back to this as I was to implement a plot-nothing fallback, I noticed that there is a PR for using gradients also for markers of lines, scatters: #1915

So maybe its worth adding it also for density?