subplot_grid with empty diagonal

[bastinat0r]

Hi, I have the following problem with subplot_grid:
I want to plot Data that is gathered in a pairwise fashion. i.e. I want to get something like

--- a>b a>c
b>a --- b>c
c>a c>b ---

with somewhat generic plots like density functions for the gathered values, boxplots etc.
However I don't know how to omit the diagonal plots where no data is available.

This results in plot(data, x=:range, xgroup=:to, ygroup=:from, Geom.subplot_grid(Geom.density)) producing the error

 ERROR: BoundsError: attempt to access 0-element Array{Int64,1}
  at index [1]
 in getindex at array.jl:282
 in apply_statistic at /home/sebastian/.julia/v0.4/Gadfly/src/statistics.jl:495
 in apply_statistics at /home/sebastian/.julia/v0.4/Gadfly/src/statistics.jl:38
 in render at /home/sebastian/.julia/v0.4/Gadfly/src/geom/subplot.jl:193
 in render_prepared at /home/sebastian/.julia/v0.4/Gadfly/src/Gadfly.jl:783
 in render at /home/sebastian/.julia/v0.4/Gadfly/src/Gadfly.jl:722
 in display at /home/sebastian/.julia/v0.4/Gadfly/src/Gadfly.jl:963
 [inlined code] from multimedia.jl:151
 in display at /home/sebastian/.julia/v0.4/Gadfly/src/Gadfly.jl:922
 in print_response at REPL.jl:134
 in print_response at REPL.jl:121
 in anonymous at REPL.jl:624

Maybe we can tell subplot_grid to omit those plots.

[bastinat0r]

This issue is quite similar to #751 – solving this would allow exactly the inted behaviour.