xeval=seq(1,3,length=25)
yeval=seq(1,2,length=25)
#create a grid of 25 x 25  = 625 values in a two column matrix
#basically each value of xeval is paired with all values of yeval to
#develop the grid.

xy=matrix(0,ncol=2,nrow=625)
 ij=0
 for(i in 1:25){
     for(j in 1:25){
      ij=ij+1
    xy[ij,1]=xeval[i]
     xy[ij,2]=yeval[j]
                }
}

#now estiimate the joint pdf at the above grid.
> fxy=((3*xy[,1])-xy[,2])/9
#since the joint pdf is estimated on a regular grid
#you can use the contour command directly
contour(xeval, yeval, t(matrix(fxy), nrow=length(xeval), ncol=length(yeval))

#or

persp(xeval, yeval, t(matrix(fxy), nrow=length(xeval), ncol=length(yeval))
#to get a perspective plot.

library(akima)      #assuming that you have the function 'akima'
#pull down  packages from the menu and select 'load packages' and follow
#instructions.

zz=interp(xy[,1],xy[,2],fxy)
contour(zz)
persp(zz)       #To get a perspective plot..