Hi,

I was trying to make a graph using trellis, and my code is:
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library(lattice)

makeGraph <- function(){

  trellis.device()
  tbg <- trellis.par.get("background")
  tbg$col <- "white"
  trellis.par.set("background",tbg)

  # Test Data
  mfr <- matrix(sample(1:100,50,replace=T),ncol=2)
  mfr <- cbind(mfr,c(rep(1,12),rep(2,13)))
  mfr <- cbind(mfr,c(rep("All",6),rep("Reduced",6),rep("All",7),rep("Reduced",6)))
  colnames(mfr) <- c("PVAL","POSITION","CHR","METHOD")
  mfr <- as.data.frame(mfr)

  pdf(file='test.pdf')
  op <- par(no.readonly=TRUE)
  xyplot(as.numeric(as.vector(PVAL)) ~ as.numeric(as.vector(POSITION)) | CHR, data = mfr, groups = METHOD, ylab = "pval", xlab = "position",
    type="p",col=c("green","red")
  )
  par(op)
  dev.off()
}

makeGraph()
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However, when I execute the code I don't get the graph. Although a pdf is generated, it is only 1 KB and when I try to open the pdf a message comes up that says there are no pages in the pdf. Interestingly, if I take the code (i.e. from line 'trellis.device()' to line 'dev.off') out of the function call and execute it, the pdf is generated just fine. 
What am I doing wrong?
thanks!

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