Thanks, Jeff,

It did work in one way if I use

xyplot(Y~X|as.factor(ID*PERIOD),data=...)

But I would like to do something like

xyplot(Y~X|as.factor(paste("ID=",ID)*paste("PERIOD=",PERIOD)),data=...)

Then, it didn't work

The error message:
Error in paste("ID=", ID) * paste("PERIOD=", PERIOD) :
  non-numeric argument to binary operator

On Tue, Feb 14, 2012 at 11:08 AM, Jeff Newmiller
<jdnewmil@dcn.davis.ca.us>wrote:

> Set up a single (factor) variable that identifies the combinations that
> exist, and plot using that variable.
> ---------------------------------------------------------------------------
> Jeff Newmiller                        The     .....       .....  Go Live...
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> Sent from my phone. Please excuse my brevity.
>
> Jun Shen <jun.shen.ut@gmail.com> wrote:
>
> >Dear all,
> >
> >In a plot command like
> >
> >xyplot(Y~X|ID*PERIOD,data=...)
> >
> >xyplot will generate all the possible ID*PERIOD combinations. But not
> >all
> >of them have data in there. So I have a lot of empty plots. How can I
> >suppress those empty plots and ask xyplot only to generate plots
> >actually
> >with data. Thanks.
> >
> >Jun
> >
> >       [[alternative HTML version deleted]]
> >
> >______________________________________________
> >R-help@r-project.org mailing list
> >https://stat.ethz.ch/mailman/listinfo/r-help
> >PLEASE do read the posting guide
> >http://www.R-project.org/posting-guide.html
> >and provide commented, minimal, self-contained, reproducible code.
>
>

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