[R] lattice auto.key drop unused levels

[Peter Ehlers]

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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