Unexpected mfrow, layout behavior (pdf still has multiple pages)

My goal is to create a graph with one column and three rows into a
single graphic. The mfrow, mfcol, and layout functions all have the
same effect: three graphs are produced in a single pdf file, having
one graph per page.  This is not what I want (I didn't want multiple
pages).  Just before I sent this post, I was able to generate the
desired output with split.screen.  Is this the expected behavior of
mfrow and layout?

Paul

--------------------------------

My code is as follows (ToruosityPlot and DiameterPlot are essentially
identical to LDRPlot).

layout(matrix(1:3, ncol=1))
#par(mfrow=c(3,1))

pdf("results.pdf", width=4, height=3, pointsize=4, colormodel="cmyk",
onefile=TRUE)

TortuosityPlot(left, right)
DiameterPlot(left, right)
LDRPlot(left, right)

dev.off()

LDRPlot <- function(left, right)
  {
    plot(left$x, left$LDR, bty="n", ann=FALSE, xlim=c(-1500, 1000),
ylim=c(1.0, 1.5))
    abline(v=0, col=gray(.90))
    lines(left$x, left$LDR, col="green4", lty="solid")
    points(left$x, left$LDR, bg="limegreen", pch=21)
    lines(right$x, right$LDR, col="blue4", lty="dashed")
    points(right$x, right$LDR, bg="blue", pch=21)
    #title(main="LDR", xlab="Distance (um)", ylab="LDR",
col.main="black", col.lab=gray(.4), cex.main=1.2, cex.lab=1.0,
font.main=4, font.lab=3)
  }


platform       i386-apple-darwin8.11.1
arch           i386
os             darwin8.11.1
system         i386, darwin8.11.1
status
major          2
minor          8.1
year           2008
month          12
day            22
svn rev        47281
language       R
version.string R version 2.8.1 (2008-12-22)