Fix legend for plotlist with multiple plots (#4546)

* Fix `legend` for plotlist with multiple plots

* Add a test

* Update CHANGELOG.md

---------

Co-authored-by: SimonDanisch <sdanisch@protonmail.com>

Author: asinghvi17

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 - Added `subsup` and `left_subsup` functions that offer stacked sub- and superscripts for `rich` text which means this style can be used with arbitrary fonts and is not limited to fonts supported by MathTeXEngine.jl [#4489](https://github.com/MakieOrg/Makie.jl/pull/4489).
+- Expand PlotList plots to expose their child plots to the legend interface, allowing `axislegend`show plots within PlotSpecs as individual entries. [#4546](https://github.com/MakieOrg/Makie.jl/pull/4546)
 - Implement S.Colorbar(plotspec) [#4520](https://github.com/MakieOrg/Makie.jl/pull/4520).
 
 ## [0.21.15] - 2024-10-25

src/makielayout/blocks/legend.jl

@@ -661,7 +661,8 @@ end
 
 function get_labeled_plots(ax; merge::Bool, unique::Bool)
     lplots = filter(get_plots(ax)) do plot
-        haskey(plot.attributes, :label)
+        haskey(plot.attributes, :label) ||
+        plot isa PlotList && any(x -> haskey(x.attributes, :label), plot.plots)
     end
     labels = map(lplots) do l
         l.label[]
@@ -713,6 +714,11 @@ function get_labeled_plots(ax; merge::Bool, unique::Bool)
 end
 
 get_plots(p::AbstractPlot) = [p]
+# NOTE: this is important, since we know that `get_plots` is only ever called on the toplevel, 
+# we can assume that any plotlist on the toplevel should be decomposed into individual plots.
+# However, if the user passes a label argument with a legend override, what do we do?
+get_plots(p::PlotList) = haskey(p.attributes, :label) && p.attributes[:label] isa Pair ? [p] : p.plots
+
 get_plots(ax::Union{Axis, Axis3}) = get_plots(ax.scene)
 get_plots(lscene::LScene) = get_plots(lscene.scene)
 function get_plots(scene::Scene)

test/specapi.jl

@@ -170,3 +170,20 @@ end
     @test isempty(f.content)
     @test isempty(f.layout.content)
 end
+
+@testset "Legend construction" begin
+    f, ax, pl = plotlist([S.Scatter(1:4, 1:4; marker = :circle, label="A"), S.Scatter(1:6, 1:6; marker = :rect, label="B")])
+    leg = axislegend(ax)
+    # Test that the legend has two scatter plots
+    @test count(x -> x isa Makie.Scatter, leg.scene.plots) == 2
+
+    # Test that the scatter plots have the correct markers
+    # This is too internal and fragile, so we won't actually test this
+    # @test leg.scene.plots[2].marker[] == :circle
+    # @test leg.scene.plots[3].marker[] == :rect
+    
+    # Test that the legend has the correct labels.
+    # Again, I consider this too fragile to work with!
+    # @test contents(contents(leg.grid)[1])[2].text[] == "A"
+    # @test contents(contents(leg.grid)[2])[4].text[] == "B"
+end