R help - May 2004 - Lattice xyplot - problem trying to produce multiple output files with a 'for' loop

I am stuck on trying to get the Lattice xyplot to output a separate PNG file
each time through my 'for' loop.  The files get produced but are empty.

Here is the code.  I'm running 1.9 on Windows.  BTW is there a more
efficient way of creating the separate output files than looping over the levels
and subsetting?

.........................................................
lev <- levels(ds$TR)

for (byvar in lev) {

 file.png <- paste("u:/data/R/Scatter_CTY_HWY_CO_TR", byvar, 
".png", sep="")

 trellis.device( device="png", file=file.png, 
     width=1000, height=1000, pointsize=20, bg="transparent" )

 ds1 <- subset(ds, TR == byvar, select=c(CTY, HWY, CO))

 xyplot( ds1$CTY ~ ds1$HWY, groups=ds1$CO,
    auto.key=list(space="right"), ylab="CTY",
xlab="HWY")

 dev.off()

}
.........................................................

--Rich

Richard Kittler 
AMD TDG
408-749-4099

richard.kittler at amd.com wrote:
> I am stuck on trying to get the Lattice xyplot to output a separate PNG
file each time through my 'for' loop.  The files get produced but are
empty.
> 
> Here is the code.  I'm running 1.9 on Windows.  BTW is there a more
efficient way of creating the separate output files than looping over the levels
and subsetting?
> 
> .........................................................
> lev <- levels(ds$TR)
> 
> for (byvar in lev) {
> 
>  file.png <- paste("u:/data/R/Scatter_CTY_HWY_CO_TR", byvar, 
".png", sep="")
> 
>  trellis.device( device="png", file=file.png, 
>      width=1000, height=1000, pointsize=20, bg="transparent" )
> 
>  ds1 <- subset(ds, TR == byvar, select=c(CTY, HWY, CO))
> 
>  xyplot( ds1$CTY ~ ds1$HWY, groups=ds1$CO,
>     auto.key=list(space="right"), ylab="CTY",
xlab="HWY")
> 
>  dev.off()
> 
> }
> .........................................................
> 
> --Rich
> 
> Richard Kittler 
> AMD TDG
> 408-749-4099
> 
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See the FAQ.

you have to wrap the xyplot call with a print. I.e.

for(...) {
   ...
   print(xyplot(...))
   ...
}