How to annotate across or between plots in multi-plot panels in R

Question

If I have several plots in a single panel, I would like to be able to put text annotations that go across more than one plot area or that go in between plot areas. I'm sure there must be a way to do this, but I need some help figuring out how.
Here's some fake data to create four plots on one panel:

x1 = rnorm(100)
x2 = rnorm(100)
x3 = rnorm(100)
x4 = rnorm(100)

par(mfrow=c(2,2)) 
hist(x1, xlab="", main="Group A")
hist(x2, xlab="", main="Group B")
hist(x3, xlab="", main="")
hist(x4, xlab="", main="")

This creates the multi-plot panel below, except that I've also added text in red. I added the red text by annotating the PDF file after I created the graph panel in R and then saved it as a PDF. What I'd like to learn is how to annotate the graph panel programmatically within R, rather than having to save it and annotate the saved file.
Normally, I would just add an annotation using text() or mtext(). But that only works if you're annotating a single graph. Is there a way to add annotations relative to the coordinates of the entire panel of 4 graphs? I'm looking for a solution in base graphics right now, but if anyone knows how to do something similar in ggplot2 or lattice, I'd be interested in seeing that as well.

Answer 1

If you truly want finer control over these kinds of layout issues, you can use the aptly named layout.

m <- matrix(c(1,2,3,3,4,5,6,6),ncol = 2,byrow = TRUE)
layout(m,widths = c(0.5,0.5),heights = c(0.45,0.05,0.45,0.05))

par(mar = c(2,4,4,2) + 0.1)
hist(x1, xlab="", main="Group A")
hist(x2, xlab="", main="Group B")

par(mar = c(0,0,0,0)) 
plot(1,1,type = "n",frame.plot = FALSE,axes = FALSE)
u <- par("usr")
text(1,u[4],labels = "Here",col = "red",pos = 1)

par(mar = c(2,4,2,2) + 0.1)
hist(x3, xlab="", main="")
hist(x4, xlab="", main="")

par(mar = c(0,0,0,0)) 
plot(1,1,type = "n",frame.plot = FALSE,axes = FALSE)
u <- par("usr")
text(1,u[4],labels = "Here",col = "red",pos = 1)