R help - Mar 2008 - adding device size-independent y=0 line to a lattice plot

Using the following lattice plot as an example, I would like to add
horizontal lines where y=0:

library(lattice)
library(grid)
fac <- gl(4,12)
x <- letters[rep(1:3,16)]
y <- runif(48,min=0.0)
dotplot(y~x|fac)

I've tried it with grid.lines using npc and native units, which works
fine unless I change the size of the output device - then the lines
are in the wrong place.  Is there a way to do this that is independent
of the output device size?

On 3/31/08, Levi Waldron <leviwaldron at gmail.com>
wrote:
> Using the following lattice plot as an example, I would like to add
>  horizontal lines where y=0:
>
>  library(lattice)
>  library(grid)
>  fac <- gl(4,12)
>  x <- letters[rep(1:3,16)]
>  y <- runif(48,min=0.0)
>  dotplot(y~x|fac)
>
>  I've tried it with grid.lines using npc and native units, which works
>  fine unless I change the size of the output device - then the lines
>  are in the wrong place.  Is there a way to do this that is independent
>  of the output device size?
The obvious thing to try would be

dotplot(y~x|fac,
        panel = function(...) {
            panel.abline(h = 0)
            panel.dotplot(...)
        })

Does this not work?

-Deepayan