Question

I would like to center a title with respect to a plot matrix (and not the overall plot) I created with this code:

d <- 3
d2 <- d*d
layout.mat <- matrix(1:d2, byrow=TRUE, ncol=d) # plot matrix
layout.mat <- cbind(layout.mat, rep(0, d)) # space
layout.mat <- cbind(layout.mat, rep(d2+1, d)) # column on the right side
wspace <- 6*par("csi")*2.54 # width of the space in character height in cm
wside <- 3*par("csi")*2.54 # width of the right side in character height in cm
layout(layout.mat, respect=TRUE, widths=c(rep(1, d), lcm(wspace), lcm(wside)))
layout.show(d2+1)
par(mar=rep(0, 4), oma=c(4,4,6,4))
for(i in 1:d){
    for(j in 1:d){
        plot.new()
        plot.window(xlim=c(0,1), ylim=c(0,1))
        ll <- par("usr")
        rect(ll[1], ll[3], ll[2], ll[4])
        text(0.5, 0.5, paste("i=",i,", j=",j,sep=""), cex=1.4)
    }
}
plot.new()
plot.window(xlim=c(0,1), ylim=c(0,1))
ll <- par("usr")
rect(ll[1], ll[3], ll[2], ll[4])
text(0.5, 0.5, "side", cex=1.4)

## title
mtext("This title should be centered according to the plot matrix", side=3,
      line=2, outer=TRUE, adj=0.5, at=0.5)

How can I determine (with given fixed distances wspace and wside) the width of plot region so that I can subtract wspace+wside from it to determine the width of the plot matrix? The ultimate goal is to determine a precise value for adj of mtext such that the title is centered above the plot matrix and not the overall plotting region.

Answer 1

How about adding this code inside your for loop:

if (i == 1 & j == 2){
   mtext("This title should be centered according to the plot matrix", side=3,
        line=2, outer=FALSE, adj=0.5, at=0.5)
}

Note the change to outer = FALSE. Here’s what it looks like, with shorter title text (the long version runs off either side in a small plot, but is still centered):

Of course, this hack only works if you have an odd number of columns. A more general solution may be to alter the layout to include a short, wide horizontal region at the top, and plot text in that region directly:

d <- 4
d2 <- d*d
layout.mat <- matrix(1:d2, byrow=TRUE, ncol=d) # plot matrix
layout.mat <- cbind(layout.mat, rep(0, d)) # space
layout.mat <- cbind(layout.mat, rep(d2+1, d)) # column on the right side
layout.mat <- rbind(c(rep(18,d),0,0),layout.mat) #Add row on top
wspace <- 6*par("csi")*2.54 # width of the space in character height in cm
wside <- 3*par("csi")*2.54 # width of the right side in character height in cm
#Note adjustments to heights
layout(layout.mat, respect=TRUE, widths=c(rep(1, d), lcm(wspace), lcm(wside)),
       heights = c(0.25,rep(1,nrow(layout.mat)-1)))
layout.show(d2+1)
par(mar=rep(0, 4), oma=c(4,4,6,4))
for(i in 1:d){
    for(j in 1:d){
        plot.new()
        plot.window(xlim=c(0,1), ylim=c(0,1))
        ll <- par("usr")
        rect(ll[1], ll[3], ll[2], ll[4])
        text(0.5, 0.5, paste("i=",i,", j=",j,sep=""), cex=1.4)
    }
}
plot.new()
plot.window(xlim=c(0,1), ylim=c(0,1))
ll <- par("usr")
rect(ll[1], ll[3], ll[2], ll[4])
text(0.5, 0.5, "side", cex=1.4)

## title
plot.new()
plot.window(xlim=c(0,1), ylim=c(0,1))
ll <- par("usr")
#rect(ll[1], ll[3], ll[2], ll[4])
text(0.5, 0.5, "top", cex=1.4)