create function to plot high density data using lattice

Dear all,
with the most useful help of Duncan I updated a script to plot high
density data in the form of 384 squares containing tiny plots. The
function works and it is possible to create a pdf version of the
output. but when i try to make a function out of this script, the
resulting pdf file is empty.
Any tips?
Thank you and best regards,
Luigi

  >>> here is the example:

DF <-  data.frame(Y = rnorm(17280),
              X = rnorm(1:45),
              Y2 = rnorm(17280)+2,
              Z  = 1:384)
plot.layout <- function(DF) {

# this works from here...
pdf(
   file = "TITLE-amp outlook.pdf",
   width = 15,
   height = 11,
   onefile = TRUE,
   family = "Helvetica",
   paper = "a4r"
)
xyplot(Y ~ X | Z,
        data = DF,
        groups = Z,
        allow.multiple = TRUE,
        ylab= "Y VALUES",
        xlab="X VALUES",
        main="TITLE",
        scales = list(
          x = list(draw = FALSE),
          y = list(draw = FALSE),
          relation="same",
          alternating=TRUE),
        as.table = TRUE,
        layout = c(24,16),
        par.settings = list(
          strip.background=list(col="white"),
          axis.text = list(cex = 0.6),
          par.xlab.text = list(cex = 0.75),
          par.ylab.text = list(cex = 0.75),
          par.main.text = list(cex = 0.8),
          superpose.symbol = list(type = "l", cex = 1)
        ),
        strip    = FALSE,
        type = "l",
        col = 3,
        panel = panel.superpose
)
dev.off()
#... till here

}
plot.layout(DF)