Sort supported args (#4156)

* sort supported args

* Update src/args.jl

* sort all supported* functions

Co-authored-by: Simon Christ <SimonChrist@gmx.de>

src/args.jl

@@ -1626,7 +1626,7 @@ function warn_on_unsupported_args(pkg::AbstractBackend, plotattributes)
         for k in sort(collect(_to_warn))
             push!(already_warned, k)
             @warn(
-                "Keyword argument $k not supported with $pkg.  Choose from: $(supported_attrs(pkg))"
+                "Keyword argument $k not supported with $pkg.  Choose from: $(join(supported_attrs(pkg), ", "))"
             )
         end
     end

src/backends.jl

@@ -285,7 +285,7 @@ for s in (:attr, :seriestype, :marker, :style, :scale)
         v = Symbol("_", bend, "_", s)
         @eval begin
             $f(::$bend_type, $s::Symbol) = $s in $v
-            $f2(::$bend_type) = $v
+            $f2(::$bend_type) = sort(collect($v))
         end
     end
 end