# [R] indexing within panels in xyplot

From: Sebastian Luque <spluque_at_gmail.com>
Date: Tue 21 Feb 2006 - 18:20:25 EST

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
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

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