Warn on extrakws

src/args.jl

@@ -1558,6 +1558,12 @@ function warn_on_unsupported_args(pkg::AbstractBackend, plotattributes)
             end
         end
     end
+    supported_extra_kws = extra_supported_kws(pkg)
+    for kw in keys(extra_kwargs)
+        if kw ∉ supported_extra_kws
+            @warn "Extra keyword argument $kw not explicitly supported with $pkg.  Choose from: $(join(supported_extra_kws, ", "))"
+        end
+    end
     extra_kwargs
 end
 

src/backends.jl

@@ -301,6 +301,7 @@ const _base_supported_args = [
     :permute,
     :unitformat,
 ]
+extra_supported_kws(pkg::AbstractBackend) = []
 
 function merge_with_base_supported(v::AVec)
     v = vcat(v, _base_supported_args)

src/backends/gr.jl

@@ -104,6 +104,8 @@ const gr_font_family = Dict(
     "computer modern" => 232,
     "dejavu sans" => 233,
 )
+extra_supported_kws(::GRBackend) =
+    [:legend_hfactor, :legend_wfactor, :nx, :ny, :display_option]
 
 mutable struct GRViewport{T}
     xmin::T

src/backends/pgfplotsx.jl

@@ -1,6 +1,7 @@
 const Options = PGFPlotsX.Options
 const Table   = PGFPlotsX.Table
 
+extra_supported_kws(::PGFPlotsXBackend) = [:add]
 Base.@kwdef mutable struct PGFPlotsXPlot
     is_created::Bool = false
     was_shown::Bool = false

src/backends/pythonplot.jl

@@ -2,6 +2,10 @@
 
 is_marker_supported(::PythonPlotBackend, shape::Shape) = true
 
+# As this gets longer perhaps it should be a Set
+extra_supported_kws(::PythonPlotBackend) =
+    [:levels, :mincnt, :edgecolors, :extend3d, :colors, :cmap, :facecolors, :shade]
+
 # problem: github.com/tbreloff/Plots.jl/issues/308
 # solution: hack from @stevengj: github.com/JuliaPy/PyPlot.jl/pull/223#issuecomment-229747768
 let otherdisplays = splice!(Base.Multimedia.displays, 2:length(Base.Multimedia.displays))