[R] xyplot does not find variable in data

[Peter Ehlers]

One other way that is sometimes useful is to use the
tick.number argument as in:

  xyplot(yield ~ nitro, data=Oats,
    scales=list(x=list(tick.number=4)), subset=Variety=="Victory"
  )

This is especially handy if you want to just tick/label
every other value.

  -Peter Ehlers

Jacob Wegelin wrote:
> When we call a lattice function such as xyplot, to what extent does
> the "data" designation cause the function to look inside the "data"
> for variables?
> 
> In the examples below, the "subset" argument understands that
> "Variety" is a variable in the data.
> 
> But the "scales" argument does not understand that "nitro" is a
> variable in the data.
> 
> What principle is at work?
> 
> 
> library(MEMSS)
> 
> # The following works fine:
> 
> xyplot(
> 	yield ~ nitro
> 	, data=Oats
> 	, scales=list(
> 		x=list(
> 			at=unique(Oats$nitro)
> 			)
> 		)
> 	, subset=Variety=="Victory"
> )
> 
> # But the following returns an error:
> 
> xyplot(
> 	yield ~ nitro
> 	, data=Oats
> 	, scales=list(
> 		x=list(
> 			at=unique(nitro)
> 			)
> 		)
> )
> 
> Thanks for any insight
> 
> 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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