Continuous color scale with Date type (Gadfly 1.2.1 vs Gadfly 1.3.1)

[riccmin]

In Gadfly 1.2.1 I was able to color points based on a continuous color scale of Date values using the following code:

using Gadfly
using Dates

xdata = collect(1:4)
ydata = collect(1:4)
cdata = collect(Date("2017-10-01"):Day(1):Date("2017-10-04"))
plot(x=xdata, y=ydata, color=cdata, Scale.color_continuous(minvalue=Date("2017-10-01"),maxvalue=Date("2017-10-08")))

This is the resulting image:

In Gadfly 1.3.1 the same code produces this error:

Error showing value of type Plot:
ERROR: There appears to be mixing of a continuous variable and a discrete variable in the `color` aesthetic.
This can happen when the color aesthetic is assigned in more than 1 layer (which is ok to do),
or you have forgotten to assign it. Either:
    1) make sure that the `color` variable in all layers is the same (i.e. discrete or continuous), or
    2) add `Scale.color_continuous` or `Scale.color_discrete` to the plot, or
    3) in a layer where `color` is a discrete variable, instead assign e.g. `color=[colorant"red"]`, or a vector of colorants.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] apply_scale(::Gadfly.Scale.ContinuousColorScale, ::Array{Gadfly.Aesthetics,1}, ::Gadfly.Data) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/scale.jl:664
 [3] apply_scales(::IterTools.Distinct{Base.ValueIterator{Dict{Symbol,Gadfly.ScaleElement}},Gadfly.ScaleElement}, ::Array{Gadfly.Aesthetics,1}, ::Gadfly.Data) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/scale.jl:24
 [4] apply_scales(::IterTools.Distinct{Base.ValueIterator{Dict{Symbol,Gadfly.ScaleElement}},Gadfly.ScaleElement}, ::Gadfly.Data) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/scale.jl:34
 [5] render_prepare(::Plot) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/Gadfly.jl:645
 [6] render(::Plot) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/Gadfly.jl:740
 [7] draw at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/Gadfly.jl:847 [inlined]
 [8] display(::Gadfly.GadflyDisplay, ::MIME{Symbol("text/html")}, ::Plot) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/Gadfly.jl:1052
 [9] display(::Gadfly.GadflyDisplay, ::String, ::Any) at ./multimedia.jl:216
 [10] display(::Gadfly.GadflyDisplay, ::Plot) at /home/ricky/.julia/packages/Gadfly/Xnkmc/src/Gadfly.jl:1009
 [11] display(::Any) at ./multimedia.jl:328
 [12] #invokelatest#1 at ./essentials.jl:710 [inlined]
 [13] invokelatest at ./essentials.jl:709 [inlined]
 [14] print_response(::IO, ::Any, ::Bool, ::Bool, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:238
 [15] print_response(::REPL.AbstractREPL, ::Any, ::Bool, ::Bool) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:223
 [16] (::REPL.var"#do_respond#54"{Bool,Bool,REPL.var"#64#73"{REPL.LineEditREPL,REPL.REPLHistoryProvider},REPL.LineEditREPL,REPL.LineEdit.Prompt})(::Any, ::Any, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:822
 [17] #invokelatest#1 at ./essentials.jl:710 [inlined]
 [18] invokelatest at ./essentials.jl:709 [inlined]
 [19] run_interface(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/LineEdit.jl:2355
 [20] run_frontend(::REPL.LineEditREPL, ::REPL.REPLBackendRef) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:1144
 [21] (::REPL.var"#38#42"{REPL.LineEditREPL,REPL.REPLBackendRef})() at ./task.jl:356

I cannot completely figure out the meaning of the error message. Am I missing something obvious? What exactly changed in Gadfly 1.3?
I am on linux MX and I am running julia 1.5.1

[tlnagy]

That error message/check was introduced by #1448 (@Mattriks). It appears that Scale.color_continuous only works for structs that subtype Number:

Gadfly.jl/src/scale.jl

Line 664 in 90185dc

isa(first(cdata), Number) || error(colorclash)

. Generally, it would be nice to support any type that supports interpolation between its values, Number or otherwise.