[R] how to make a contour plot in R?

[Arnau Mir Torres]

Hello.

I am an nxn data frame in the variable frame.
I want to make a contour plot with it. That is, I want to plot a square 
of dimensions [1,n]x[1,n] with the  gray level of square [i,i+1]x[j,j+1] 
equal to frame[i,j].

How can I make it?

Thanks,

Arnau.