[R] help with plotting a grid on levelplot

[Deepayan Sarkar]

On Tuesday 29 March 2005 19:16, Jeff D. Hamann wrote:
> 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" )

You are not allowed to use base graphics functions (like grid) in 
lattice panel functions. You have to use grid (the package, not the 
function) functions. There are many such functions defined in lattice 
which can be useful, in this case 'panel.grid' or more likely 
'panel.abline'.

I'm not sure I understand your attempted usage though; you seem to 
assign the value of grid() (which would be NULL) to a variable called 
panel.last. This does not have any possible use that I can see.

>                  }
>                  ) )
>
> 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.

Unless you understand very well what you are doing, I would stay away 
from micro-managing calls to complicated panel functions like 
panel.levelplot. In particular, I would suggest something like

panel = function(...) {
  panel.levelplot(...)
  panel.abline(h=<whatever>, v=<whatever>)
}


> 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
>
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