Order dependent incorrect handling of NaN in Geom.ribbon

[tlnagy]

Ran into a bug with Geom.ribbon where if ymin/ymax are first NaN then concrete values, the concrete values are ignored. However, if the concrete values appear first then it works correctly.

using Gadfly
srand(123);
ys = rand(20)
plot(x=repeat(1:10, outer=2), y=ys, ymin=ys.-1, ymax=ys.+1, color=repeat([:a,:b], inner=10), Geom.line, Geom.ribbon)

Concrete values first

ys2 = copy(ys)
ys2[11:end] = NaN
plot(x=repeat(1:10, outer=2), y=ys2, ymin=ys2.-1, ymax=ys2.+1, color=repeat([:a,:b], inner=10), Geom.line, Geom.ribbon)

NaN values first

The expect behavior here is Geom.ribbon showing the yellow values

ys2 = copy(ys)
ys2[1:10] = NaN
plot(x=repeat(1:10, outer=2), y=ys2, ymin=ys2.-1, ymax=ys2.+1, color=repeat([:a,:b], inner=10), Geom.line, Geom.ribbon)