R help - Feb 2009 - xyplot multiple conditions and empty panels

Dear R-users,

Is it possible to prevent xyplot from drawing empty panels when using 
multiple conditions and when the dataset has a incomplete set of 
condition combinations - like in the following example?
I have tried to modify the drop.unused.levels argument but without any 
success.
Any help and suggestions will be appreciated.

Sebastien

####
df <- data.frame(id=c(1,1,2,2,1,1,3,3,4,4),
                 grp=c(1,1,1,1,2,2,2,2,2,2),
                 x=1:10,
                 y=1:10,
                 z=jitter(1:10))
xyplot(y+z~x|id*grp,
       data=df,
       type=c("p","l"),
       distribute.type = T,
       pch=3,
       col=c(1,2),
       xlab="x axis",
       ylab="y axis")
####

On Fri, Feb 6, 2009 at 1:15 PM, Sebastien Bihorel
<Sebastien.Bihorel at cognigencorp.com> wrote:
> Dear R-users,
>
> Is it possible to prevent xyplot from drawing empty panels when using
> multiple conditions and when the dataset has a incomplete set of condition
> combinations - like in the following example?
> I have tried to modify the drop.unused.levels argument but without any
> success.
> Any help and suggestions will be appreciated.
>
> Sebastien
>
> ####
> df <- data.frame(id=c(1,1,2,2,1,1,3,3,4,4),
>                grp=c(1,1,1,1,2,2,2,2,2,2),
>                x=1:10,
>                y=1:10,
>                z=jitter(1:10))
> xyplot(y+z~x|id*grp,
>      data=df,
>      type=c("p","l"),
>      distribute.type = T,
>      pch=3,
>      col=c(1,2),
>      xlab="x axis",
>      ylab="y axis")
> ####
Levels are dropped for individual terms, not their combinations. If
you mean to use the combination as a term, do so explicitly:

xyplot(y+z~x | interaction(id, grp), df)

-Deepayan