Plotting a surface in Julia
In this case the simplest approach is to use a wrapper anonymous function:
(x...) -> f(x)
that you can pass to the plotting function instead of your original function.
Now you have:
julia> f(x)=x[1]-x[2]
f (generic function with 1 method)
julia> ((x...) -> f(x))(100, 5)
95