Add xstd aesthetics to errobar ?

[jonathanBieler]

Typically when using Geom.errorbar you end up doing something like :

y = y, ymin = y .- ysd, ymax = y .+ ysd

Which is a bit tedious, I think it would be nice to add a x_std aesthetics such that you could do :

y =  y, y_std = sd

instead.

If you agree and it's not to hard to do I could try to prepare a PR.

[Mattriks]

Similar to #1307. Would you use the Measurements.jl package, if Gadfly supported it?

[jonathanBieler]

That would be even better yes, if that can be implemented without the stat.measurement part (i.e. just plot(x=1:10, y=y, Geom.point, Geom.errorbar)).