Problem printing greyscale or B&W ggplot2 images

Question

I have an issue with printing/creating black and white and greyscale images with ggplot2.

I have been trying to make some black and white, and greyscale graphics for publication, they look perfect on mine and everyone's monitors, but came out in colour when printed. (An awful photo of the result is here: http://i.imgur.com/UuB6A.jpg)

For example:

p <- qplot(displ, hwy, data = mpg)
p <- p + facet_wrap(~ cyl) + theme_bw()

 pdf("testplot.pdf",width=7.6,height=3.6)
     p
 dev.off()

The generated image and output pdf looks perfect onscreen, but the facets borders and titles are in a shade of red when printed. Try it!

I get the same results when outputting to a .png, and also when I print directly from the R graphics window. According to my friend with some experience with image manipulation, the images I've sent him have bits of colour in them according to the colour histogram.

Is there any way to generate true B&W and greyscale images using ggplot? I tried forcing greyscale (perhaps very naively!) by using 'colormodel="gray"' in the pdf() command, but it does not make a difference.

Any help would be greatly appreciated.

Answer 1

Printing ggplot2 plots to the png or jpeg devices does result in RGB color space files, like you describe. If you need a true grayscale color model for those, they can easily be coverted with photoshop droplets or similar.

For the pdf device, however, the grays in ggplot2 figures are being correctly described as grayscale black percentages (i.e. 20% black for the facet labels background, 80% black for the histogram bars, 10.2% black for the main background) even without the colormodel argument.

require(ggplot2)

n = 1000

group = factor(rep(1:3, len=n))
value = as.numeric(group) + rnorm(n)

data = data.frame(group, value)

dev.new(width=5, height=2)
p = qplot(value, data=data) + facet_wrap(~group, nrow=1)
p

dev.print(device=png, file='test.png', width=dev.size()[1], units='in', res=100)
dev.print(device=pdf, file='test.pdf')