Access to conditioning variables (lattice)

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Martin D. Lepage
Sent: Thursday, September 24, 2009 7:38 AM
To: r-help at r-project.org
Subject: [R] Access to conditioning variables (lattice)

[using R version 2.8.1 (2008-12-22)]

Hello,

I'm trying to access the conditioning variables of an xyplot within a
'panel' function but I have not been able to figure out how to do so.
Here is a simple example that describes what I wish to do (the problem
lies with the commented line):

dataset <- data.frame(x = c(1,2), y = c(4,5), Type = 
factor(c("a","b")))
xyplot( y ~ x | Type, dataset, 
        panel = function(...) {
            panel.xyplot(...)
#           do_something_with(conditioning_variables[which.packet()])
        })

The problem I am facing is that I do not know how to 
generically access
the conditioning variables within the panel function. In this simple
case, I can achieve what I want to do with the following call :

    do_something_with(Type[which.packet()])

but that requires the panel function call to have prior 
knowledge of the
object used as the conditioning variable, which is not flexible enough
for my needs.

Thank you,
Martin D. Lepage