add stronger typing to poetic plotting, fixes #882

[tlnagy]

Related to #881 and #871

[coveralls]

Coverage increased (+0.2%) to 65.67% when pulling 47336cb on tlnagy:pull-request/24f5d936 into 4a36837 on dcjones:master.

[coveralls]

Coverage increased (+0.02%) to 65.458% when pulling 47336cb on tlnagy:pull-request/24f5d936 into 4a36837 on dcjones:master.

[shashi]

Wow this is a pretty action at a distance kind of problem. I guess this fix is warranted though.

[tlnagy]

Indeed. I would've preferred a different workaround to this (something more like #874), but that was a lucky fix. However, this is a much more robust fix for this error and it looks like the tests pass. Hopefully no one is passing anything too funky to the a and b parameters.

Also, based on your suggestion in #882, I added a test for the ambiguity method error.

[coveralls]

Coverage increased (+0.02%) to 65.458% when pulling 8804dde on tlnagy:pull-request/24f5d936 into 4a36837 on dcjones:master.

[coveralls]

Coverage increased (+0.02%) to 65.458% when pulling 8804dde on tlnagy:pull-request/24f5d936 into 4a36837 on dcjones:master.

[shashi]

Hopefully no one is passing anything too funky to the a and b parameters.

julia> brightness(x::RGB) = (x.r+x.g+x.b)/3
brightness (generic function with 1 method)

julia> plot([brightness], colorant"black", colorant"white")
ERROR: MethodError: `isless` has no method matching isless(::ColorTypes.RGB{FixedPointNumbers.UFixed{UInt8,8}}, ::ColorTypes.RGB{FixedPointNumbers.UFixed{UInt8,8}})
Closest candidates are:
  isless(::DataArrays.NAtype, ::Any)
  isless(::Any, ::DataArrays.NAtype)
 in plot at /home/shashi/.julia/v0.4/Gadfly/src/poetry.jl:44

haha.

yeah, if someone's using funky a and b then they can open an issue.