plotting image/contour on irregular grid

Hello,
I'd like to make a z(x,y) plot for irregularly spaced
x,y. What are routines are available in R for this
purpose? 

One possibility is to interpolate a regular grid using interp() in the 
akima package, then use image() or contour(). Another is to use 
levelplot() with formula z ~ x + y in the lattice package, and the 
equivalent contourplot(); here, the x,y pairs must lie on a grid, but do 
not need to fill the grid (so are regularly spaced with missing grid 
cells).

Thanks,
Mark

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