R help - Oct 2009 - lattice auto.key drop unused levels

The following code produces a legend ("key") that mentions the unused
levels of Block.

library(MEMSS)
xyplot(yield~nitro, subset=(Block=="I" | Block=="II"),
data=Oats,
group=Block, auto.key=T)

and adding "drop.unused.levels=T" does not fix it. And in fact even
the following does not solve the problem:

 xyplot(yield~nitro, data=Oats[Oats$Block=="I" |
Oats$Block=="II",],
group=Block, auto.key=T)

The following workaround solves it, but seems inelegant:

junk<-Oats[Oats$Block=="I" | Oats$Block=="II",]
junk$Block<-factor(as.character(junk$Block))
xyplot(yield~nitro, group=Block, data=junk, auto.key=T)

What is the elegant or "proper R thinking" way to do this? That is, I
want to get a key that only mentions the levels of Block that are used
in the plot.

Thanks

Jacob A. Wegelin
Assistant Professor
Department of Biostatistics
Virginia Commonwealth University
730 East Broad Street Room 3006
P. O. Box 980032
Richmond VA 23298-0032
U.S.A.
E-mail: jwegelin at vcu.edu
URL: http://www.people.vcu.edu/~jwegelin

The key will show the levels of the 'groups' factor. So you will
have to ensure that the factor fed to groups has the levels
that you want displayed. ?xyplot explicitly states that
drop.unused.levels will NOT do that for you.

(Didn't Deepayan just answer something like this?)

  -Peter Ehlers

Jacob Wegelin wrote:
> The following code produces a legend ("key") that mentions the
unused
> levels of Block.
> 
> library(MEMSS)
> xyplot(yield~nitro, subset=(Block=="I" | Block=="II"),
data=Oats,
> group=Block, auto.key=T)
> 
> and adding "drop.unused.levels=T" does not fix it. And in fact
even
> the following does not solve the problem:
> 
>  xyplot(yield~nitro, data=Oats[Oats$Block=="I" |
Oats$Block=="II",],
> group=Block, auto.key=T)
> 
> The following workaround solves it, but seems inelegant:
> 
> junk<-Oats[Oats$Block=="I" | Oats$Block=="II",]
> junk$Block<-factor(as.character(junk$Block))
> xyplot(yield~nitro, group=Block, data=junk, auto.key=T)
> 
> What is the elegant or "proper R thinking" way to do this? That
is, I
> want to get a key that only mentions the levels of Block that are used
> in the plot.
> 
> Thanks
> 
> Jacob A. Wegelin
> Assistant Professor
> Department of Biostatistics
> Virginia Commonwealth University
> 730 East Broad Street Room 3006
> P. O. Box 980032
> Richmond VA 23298-0032
> U.S.A.
> E-mail: jwegelin at vcu.edu
> URL: http://www.people.vcu.edu/~jwegelin
> 
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