Multiple plot in a single image using ImageMagick

Sometimes you need to add several plots/images either by row or by column to a single page/sheet.
If you generate all your plot with R base graphics you can easily accomplished the task using the par() function, e.g., using par(mfrow=c(2,2)) and then drawing 4 plots of your choice.
However, if you need to create a single image build up from different sources, e.g. external images, plots not compatible with R base graphics, etc. , you can create/retrieve the single images and then merge them together using the tools from the Unix (Linux, Mac OS X, etc.) ImageMagick suite.

## Example
# we generate some random plot
require(seqLog)
## the first plot is taken from the seqLogo help ( ?seqLogo )
## I selected this example on purpose because the seqLogo function is based on the grid graphics
and is coded in such a way that doesn't allow the use of the par() function
mFile <- system.file("Exfiles/pwm1", package="seqLogo")
m <- read.table(mFile)
pwm <- makePWM(m)
png("seqLogo1.png", width=400, height=400)
seqLogo(pwm)
dev.off()
## totally unrelated
png("plot1.png", width=400, height=400)
plot(density(rnorm(1000)))
dev.off()

Then you can type:

system("convert \\( seqLogo1.png plot1.png +append \\) \\( seqLogo1.png plot1.png +append \\) -background none -append final.png")

Remember that in R you have to start escape character with '\' !

Or, alternatively, from the command line:

convert \( seqLogo1.png plot1.png +append \) \( seqLogo1.png plot1.png +append \) -background none -append final.png

See man convert and man ImageMagick for the full story.