Exception in apply_scales() when using multiple layers+DataFrame+color keys

[azurefx]

using Gadfly
using DataFrames
f(x,y)=x^2+y^2
xs=-1:0.01:1
ys=-1:0.01:1
mat=[
    0 0 "A";
    0.5 0.5 "A";
    0 0 "B";
    0.5 -0.5 "B";
    0 0 "C";
    -0.5 0.5 "C"
]
df=DataFrame(x=Float64[],y=Float64[],series=String[])
for i in indices(mat,1)
    push!(df,mat[i,:])
end
categorical!(df,:series)

#OK
plot(
    layer(x=xs,y=ys,z=f,Geom.contour)
)
#OK
plot(
    layer(df,x=:x,y=:y,color=:series)
)
#OK
plot(
    layer(x=xs,y=ys,z=f,Geom.contour),
    layer(df,x=:x,y=:y)
)
#FAILED
plot(
    layer(x=xs,y=ys,z=f,Geom.contour),
    layer(df,x=:x,y=:y,color=:series)
)

Error showing value of type Gadfly.Plot: ERROR: MethodError: Cannotconvertan object of type String to an object of type Float64 This may have arisen from a call to the constructor Float64(...), since type constructors fall back to convert methods. Stacktrace: [1] convert(::Type{Float64}, ::CategoricalArrays.CategoricalString{UInt32}) at C:\Users\Azure\.julia\v0.6\CategoricalArrays\src\value.jl:67 [2] apply_scale(::Gadfly.Scale.ContinuousColorScale, ::Array{Gadfly.Aesthetics,1}, ::Gadfly.Data, ::Vararg{Gadfly.Data,N} where N) at C:\Users\Azure\.julia\v0.6\Gadfly\src\scale.jl:546 [3] apply_scales(::IterTools.Distinct{Base.ValueIterator{Dict{Symbol,Gadfly.ScaleElement}},Gadfly.ScaleElement}, ::Array{Gadfly.Aesthetics,1}, ::Gadfly.Data, ::Vararg{Gadfly.Data,N} where N) at C:\Users\Azure\.julia\v0.6\Gadfly\src\scale.jl:34 [4] apply_scales(::IterTools.Distinct{Base.ValueIterator{Dict{Symbol,Gadfly.ScaleElement}},Gadfly.ScaleElement}, ::Gadfly.Data, ::Gadfly.Data, ::Vararg{Gadfly.Data,N} where N) at C:\Users\Azure\.julia\v0.6\Gadfly\src\scale.jl:53 [5] render_prepare(::Gadfly.Plot) at C:\Users\Azure\.julia\v0.6\Gadfly\src\Gadfly.jl:677 [6] render(::Gadfly.Plot) at C:\Users\Azure\.julia\v0.6\Gadfly\src\Gadfly.jl:755 [7] show at C:\Users\Azure\.julia\v0.6\Gadfly\src\Gadfly.jl:959 [inlined] [8] verbose_show(::Base.AbstractIOBuffer{Array{UInt8,1}}, ::MIME{Symbol("image/svg+xml")}, ::Gadfly.Plot) at .\multimedia.jl:42 [9] #sprint#228(::Void, ::Function, ::Int64, ::Function, ::MIME{Symbol("image/svg+xml")}, ::Vararg{Any,N} where N) at .\strings\io.jl:66 [10] reprmime(::MIME{Symbol("image/svg+xml")}, ::Gadfly.Plot) at .\multimedia.jl:61 [11] stringmime at .\multimedia.jl:83 [inlined] [12] display(::_vscodeserver.InlineDisplay, ::MIME{Symbol("image/svg+xml")}, ::Gadfly.Plot) at C:\Users\Azure\.vscode\extensions\julialang.language-julia-0.10.2\scripts\terminalserver\terminalserver.jl:167 [13] display(::_vscodeserver.InlineDisplay, ::Gadfly.Plot) at C:\Users\Azure\.vscode\extensions\julialang.language-julia-0.10.2\scripts\terminalserver\terminalserver.jl:198 [14] display(::Gadfly.Plot) at .\multimedia.jl:218 [15] hookless(::Media.##7#8{Gadfly.Plot}) at C:\Users\Azure\.julia\v0.6\Media\src\compat.jl:14 [16] render(::Media.NoDisplay, ::Gadfly.Plot) at C:\Users\Azure\.julia\v0.6\Media\src\compat.jl:27 [17] display(::Media.DisplayHook, ::Gadfly.Plot) at C:\Users\Azure\.julia\v0.6\Media\src\compat.jl:9 [18] display(::Gadfly.Plot) at .\multimedia.jl:218 [19] eval(::Module, ::Any) at .\boot.jl:235 [20] print_response(::Base.Terminals.TTYTerminal, ::Any, ::Void, ::Bool, ::Bool, ::Void) at .\REPL.jl:144 [21] print_response(::Base.REPL.LineEditREPL, ::Any, ::Void, ::Bool, ::Bool) at .\REPL.jl:129 [22] (::Base.REPL.#do_respond#16{Bool,_vscodeserver.##3#4,Base.REPL.LineEditREPL,Base.LineEdit.Prompt})(::Base.LineEdit.MIState, ::Base.AbstractIOBuffer{Array{UInt8,1}}, ::Bool) at .\REPL.jl:646

[Mattriks]

See #1098 and my #1098 (comment)

[azurefx]

Thanks for your workaround! I hope this can be supported in the future.