>> What principle is at work?
>
> A strange one called "standard non-standard evaluation"; see
>
> http://developer.r-project.org/nonstandard-eval.pdf
>
> for a nice overview by Thomas Lumley.
>
>
> ?xyplot says:
>
> data: For the 'formula' method, a data frame containing values (or
> more precisely, anything that is a valid 'envir' argument in
> 'eval', e.g. a list or an environment) for any variables in
> the formula, as well as 'groups' and 'subset' if applicable.
> If not found in 'data', or if 'data' is unspecified, the
> variables are looked for in the environment of the formula.
> For other methods (where 'x' is not a formula), 'data' is
> usually ignored, often with a warning.
>
> so the non-standard evaluation only applies to 'groups' and 'subset'.
> The list may be different for other functions, e.g., densityplot()
> also evaluates 'weights' in 'data'.
>
> -Deepayan
In addition, you might want to use with, as in:
with( Oats, xyplot(
yield ~ nitro,scales=list(x=list(at=unique(nitro))),
subset=Variety=="Victory"
) )
Romain
-- which standardizes the standard non-standardized evaluation I guess :-).
Bert
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