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?

 The obvious thing to try would be

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

 Does this not work?

 -Deepayan