Incorrect behavior when color vectors of intermediate lengths passed

[tlnagy]

While working on #1152, I noticed some troublesome behavior related to the usage of cycle for the color aesthetic. It occurs when the length of the color vector is > 1 and < n where n is the length of the other aesthetics.

length(color) == n

using RDatasets, Gadfly
data = dataset("datasets", "HairEyeColor")
plot(data, x=:Freq, color=:Sex, Geom.density)

This is the correct behavior

length(color) == 1

plot(data, x=:Freq, color=[colorant"deepskyblue"], Geom.density)

Also correct behavior

1 < length(color) < n

plot(data, x=:Freq, color=[colorant"deepskyblue", colorant"orange"], Geom.density)

This looks really weird. It turns out that what is happening is happening is that it assigns the first x value to blue, the next to orange, followed by blue, etc. I don't think there is any reason that the user would predict this behavior. Here's where it happens:

Gadfly.jl/src/statistics.jl

Lines 511 to 516 in f28c02c

	for (x, c) in zip(aes.x, cycle(aes.color))
	if !haskey(groups, c)
	groups[c] = Float64[x]
	else
	push!(groups[c], x)
	end

Other geometries (Geom.line, etc) prohibit the color vector from having a different dimension from the other aesthetics. I think this is a safer approach, even if it gets rid of the convenience situation of length(color) == 1. Our other option is enforce that length(color) == 1 or == n for all geometries. I think we should be consistent either way.

Also, it looks like several other geometries (notably Geom.histogram and Geom.boxplot) also make this same mistake