[R] indexing within panels in xyplot

[Sebastian Luque]

Dear R-helpers,

I need to show a linear fit through a subset of the data within each
combination of levels of two factors.  So I prepared an xyplot with
different panels for each level of one of the factors, and different
symbols within each panel for the levels of the second factor.  My problem
is selecting the subset of each combination through which the line should
be fit for subsequent plotting.  This hopefully shows the idea:


---<---------------cut here---------------start-------------->---
toydf <- expand.grid(1:100, c("A", "B"),
                     c("pop1", "pop2", "pop3", "pop4", "pop5"))
toydf <- data.frame(facA = toydf[[3]], facB = toydf[[2]],
                    x = toydf[[1]], y = rnorm(1000))

xyplot(y ~ x | facA, groups = facB, data = toydf,
       panel.groups = function(x, y, subscripts, ...) {
         panel.xyplot(x, y, ...)
         lindx <- which(y[subscripts] == max(y[subscripts], na.rm = TRUE))
         xleft <- mean(x[lindx], na.rm = TRUE)
         fit <- lm(y[x >= xleft] ~ x[x >= xleft])
         panel.abline(fit)
       })
---<---------------cut here---------------end---------------->---

i.e. the left limit for fitting the line is defined by the mean of x
values where y is equal to the maximum y values, *within* each combination
of levels of both factors.  The above is giving me:

Error in lm.fit(x, y, offset = offset, singular.ok = singular.ok, ...) : 
	0 (non-NA) cases
In addition: Warning message:
no finite arguments to max; returning -Inf 

which shows I'm not understanding how the 'subscripts' argument works.
I'd appreciate some pointers on what I'm doing wrong, as I haven't been
able to find help in the help pages and List archives.

Thanks,

-- 
Sebastian P. Luque