add test

src/args.jl

@@ -656,7 +656,7 @@ function default(k::Symbol)
         return _axis_defaults_byletter[letter][key]
     end
     k == :letter && return k # for type recipe processing
-    k in _suppress_warnings || error("Unknown key: ", k)
+    return missing
 end
 
 function default(k::Symbol, v)
@@ -1133,7 +1133,7 @@ function warn_on_unsupported_args(pkg::AbstractBackend, plotattributes)
         k in _suppress_warnings && continue
         default_value = default(k)
         if ismissing(default_value)
-            extra_kwargs[k] = pop!(plotattributes, k)
+            extra_kwargs[k] = pop_kw!(plotattributes, k)
         elseif plotattributes[k] != default(k)
             k in already_warned || push!(_to_warn, k)
         end

src/pipeline.jl

@@ -341,8 +341,8 @@ function _expand_subplot_extrema(sp::Subplot, plotattributes::AKW, st::Symbol)
 end
 
 function _add_the_series(plt, sp, plotattributes)
-    plt[:extra_kwargs][:series] = warnOnUnsupported_args(plt.backend, plotattributes)
-    warnOnUnsupported(plt.backend, plotattributes)
+    plt[:extra_kwargs][:series] = warn_on_unsupported_args(plt.backend, plotattributes)
+    warn_on_unsupported(plt.backend, plotattributes)
     series = Series(plotattributes)
     push!(plt.series_list, series)
     push!(sp.series_list, series)

test/test_pgfplotsx.jl

@@ -326,3 +326,9 @@ end
       #    end
    end # testset
 end # testset
+
+@testset "Extra kwargs" begin
+   pl = plot(1:5, test = "me")
+   @test pl.attr[:extra_kwargs][:series][:test] == "me"
+
+end # testset