title: Plot as bitmap in PDF
body: <p>I am currently working on CGH array results, which involve several plots of dozens of thousands of points, and i would like to benefit from the multiple page feature of the PDF device and the lightness of the PNG image format.</p>
  
  <p>The problem is that the PDF device stores the plots as vectorial drawings, so the PDF files are huge and take several minutes to open. I wonder if R can plot as multiple bitmaps embedded in a single PDF file, as i know the PDF format able to handle it.</p>
  
  <p>Here is a simple example, the PDF file is about 2 Mo while the png ones are about 10 Ko, so I'd like a PDF file of about 20 Ko.</p>
  
  <pre><code>png("test%i.png")
  plot(rnorm(2e4), rnorm(2e4), pch="+", cex=0.6)
  plot(rnorm(2e4), rnorm(2e4), pch="+", cex=0.6)
  dev.off()
  
  pdf("test.pdf", onefile=TRUE)
  plot(rnorm(2e4), rnorm(2e4), pch="+", cex=0.6)
  plot(rnorm(2e4), rnorm(2e4), pch="+", cex=0.6)
  dev.off()
  </code></pre>