I wish to identify groups representing different treatments, but to plot 
them and do a regression using a continuous variable ("cover") 
ignoring the groupings.

d$year <- NA
d$year <-c(rep(2007,12), rep(2008,12))
d$treatment <- c(rep("A",4),rep("B",4),rep("C",4), rep("A",4), rep("B",4), 
rep("C",4))
d$cover <- rnorm(24)
d$variable <- rnorm(24)

xyplot(variable ~ cover | year, d, 
        type=c("p","r"),
        groups=treatment
        )

As it stands, a different regression line is plotted for each treatment.
Oh, and how do I display the actual numeric value of year (e.g., "2007") 
in the strip, rather than the word "year"?

--Seth



Dr. Seth  W. Bigelow
Biologist, USDA-FS Pacific Southwest Research Station
1731 Research Park Drive, Davis California
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