Plots.jl: interrogating Plot

Dear Plots experts—I want to write a function that calculates the native slope between two points, so that I can put a label there with the same angle. So, I need to find out, from the plot object, what the four corners (absolute ([0,0], [0,1], [1,0], [1,1]) correspond to in graph space. I also need the aspect ratio. For comparison with R, I need par("plt") and par("usr"). I also want to know where to place the label so that it is 1em to the x=left of a point. Same inputs needed.

I first went to dump, but this also had some odd output:

julia> dump( plot( 1:10, 11:20 ) )
  subplots: Array{Plots.Subplot}((1,))
    1: Plots.Subplot{Plots.GRBackend}
      parent: Plots.GridLayout
        parent: Plots.RootLayout ERROR: MethodError: no method matching size(::Plots.RootLayout)
Closest candidates are:
  size(::BitArray{1}) at bitarray.jl:70
  size(::BitArray{1}, ::Any) at bitarray.jl:74
  size(::Core.Compiler.StmtRange) at show.jl:1561

pointers appreciated.