[R] help with plotting a grid on levelplot

[Jeff D. Hamann]

I'm trying to plot a grid over a levelplot

print( levelplot( var1.pred~x+y,
                 data=saw.pred,
                 aspect=mapasp(saw.pred),
                 col.regions=terrain.colors(80),
                 main=main) )

using the data...

> saw.pred
      x   y var1.pred var1.var sort
1     5   5 3.3761200 256.3363  saw
2    15   5 3.3884142 499.5695  saw
3    25   5 3.5394769 362.5490  saw
4    35   5 3.6386983 439.3200  saw
5    45   5 3.1866799 458.5570  saw
6    55   5 2.8838555 331.2363  saw
..blah...blah...blah...
15  145   5 0.8705088 256.3363  saw
16    5  15 3.1183053 455.3765  saw
17   15  15 3.2780465 443.1954  saw
18   25  15 3.7146268 435.1166  saw

which looks just fine. I know want to plot a grid such that the cells are 
grouped into 5x5 cells (or some other grid spacing) over the levelplot for a 
demonstration and I'm getting stuck when trying to modify my levelplot() 
call above to include the extra calls to panel.levelplot...

I've gotten this far,

## plot the results (10mx10m plot that's 150m x 200m)
main=expression( paste( widehat(V)[saw], ", in ", m^3, ha^{-1} ) )
print( levelplot( var1.pred~x+y,
                 data=saw.pred,
                 aspect=mapasp(saw.pred),
                 main=main,
                 panel = function(x,y,z=var1.pred )
                 {
                   panel.levelplot( x=x, y=y, z=z,
                                   subscripts=1:nrow(saw.pred),
                                   col.regions=terrain.colors(80),
                                   )
                   panel.last=grid( 15, 20, lwd=2, lty="solid", 
col="black" )
                 }
                 ) )

and the legend does match the same colors as the levelplot and the grid 
isn't centered on the levelplot either.

Could someone give me some direction? I've stayed away from modifying the 
panel.XX args when I use lattice, and now I need help.

Thanks,
Jeff.

---
Jeff D. Hamann
Forest Informatics, Inc.
PO Box 1421
Corvallis, Oregon USA 97339-1421
541-754-1428
jeff.hamann at forestinformatics.com
www.forestinformatics.com