I wish to create a multipanel plot (map) from several datasets ("d" and 
"q" in the example below). I can condition the main xyplot statement on 
the "site" variable, but I don't know how to pass a conditioning variable 
to panel.xyplot plot so that the x-y coordinates from dataset q are only 
plotted at the appropriate site.

 
library(lattice)
d <- data.frame(site  = c(rep("A",12), rep("B",12)), 
x=rnorm(24),y=rnorm(24))                        # Create dataframe "d", 
with 12 x-y coordinates for each site
q <- data.frame(site  = c(rep("A",7), rep("B",7)), 
x=rnorm(14),y=rnorm(14))                        # Create dataframe "q", 
with 7 pairs of x-y coordinates for each site.

mypanel <- function(...){
        panel.xyplot(q$x, q$y, col="red")               # Statement that 
needs a "Site" conditioning variable
        panel.xyplot(...)}  

xyplot(y~x|site, d, panel=mypanel)      # Statement erroneously plots all 
14 x-y points in "q" on panels for sites A & B
 


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