It is well-known that R has several graphics devices – either the screen devices (X11(), windows(), …) or the off-screen devices (pdf(), png(), …). We can query the default graphics device in options():
getOption('device')
In a non-interactive session, the default device is pdf(). This is why Sweave has to create a file named Rplots.pdf no matter if you want it or not when you run Sweave on an Rnw file which has code chunks creating plots. Such a behaviour is annoying to me – the PDF file is not only unnecessary, but also time-consuming (creating this PDF file is completely a waste of time). Is there a way to set a “null” device? (like the /dev/null for *nix users) The answer is yes, but not so obvious. I have not found the device below documented anywhere:
options(device = function(...) {
.Call("R_GD_nullDevice", PACKAGE = "grDevices")
})
This device can speed up Sweave a lot when there are many plots to draw. Here is a comparison:
x = rnorm(1000)
system.time({
.Call("R_GD_nullDevice", PACKAGE = "grDevices")
replicate(500, plot(x, pch = 1:21))
dev.off()
})
# user system elapsed
# 1.51 0.02 1.53
system.time({
pdf(file.path(tempdir(), "Rplots.pdf"))
replicate(500, plot(x, pch = 1:21))
dev.off()
})
# user system elapsed
# 47.81 0.20 48.10
One thing I don’t understand in Sweave is that it evaluates the code chunk twice if its Sweave options contain fig=TRUE. I think this might be a waste of time as well, and this is why I like pgfSweave, which has both the mechanism of caching R objects (using cacheSweave) and a smart way to cache graphics (using pgf).
WARNING: this null device may not work with plots that contain (math) expressions! (take a look at demo(plotmath) in case you do not know what are expressions in R graphics)