Title key on each panel of a plot generated with par(mfrow=c(x,y))

Question

For use in a scientific publication I would like to add a key (A, B, C) as a 'title' to three plots in one graph generated with par(mfrow=c(3,1)). A very easy solution to add a nice 'title' key to a single plot is to use the 'outer' parameter of the title() (e.g. title(outer=T,adj=0,main="A",cex=1.1, col="black",font=2,line=-1)) however when this would be used for multiple plots, all titles end up in the same place:

par(mfrow=c(3,1))
plot(rnorm(100),col="red")
title(outer=T,adj=0,main="A",cex=1.1,col="black",font=2,line=-1)
plot(rnorm(100),col="blue")
title(outer=T,adj=0,main="B",cex=1.1,col="black",font=2,line=-1)
plot(rnorm(100),col="green")
title(outer=T,adj=0,main="C",cex=1.1,col="black",font=2,line=-1)

I am aware that I could use layout and/or mtext (Common main title of a figure panel compiled with par(mfrow)) however I feel there might be an easier solution that I am overlooking (I have no acces to Adobe Illustrator and would like to use R to generate the graph completely). The solution I can get with the use of layout is not good enough (key is too much separated from the plot and not enough to the left), also I find the playing around with the margins and coordinates a bit too tedious.

par(mar=c(1,2,2,1))
layout(matrix(seq(1,6),ncol=1),heights=rep(c(1,3),3))
plot.new()
text(0,0.5,"A",cex=1.1,font=2)
plot(rnorm(100),col="red")
plot.new()
text(0,0.5,"B",cex=1.1,font=2)
plot(rnorm(100),col="blue")
plot.new()
text(0,0.5,"C",cex=1.1,font=2)
plot(rnorm(100),col="blue")

Any help would be much appreciated. Even if the answer would be that this is simply not possible in an easy way, that would still be very helpful.

Answer 1

When you use outer=TRUE, you are asking to write the title in the outer margin (common to all sub-plots). To do what you want, just set outer=FALSE:

outer = FALSE

line = -2
cex = 2
adj  = 0.025

par(mfrow=c(3,1))
plot(rnorm(100),col="red")
title(outer=outer,adj=adj,main="A",cex.main=cex,col="black",font=2,line=line)
plot(rnorm(100),col="blue")
title(outer=outer,adj=adj,main="B",cex.main=cex,col="black",font=2,line=line)
plot(rnorm(100),col="green")
title(outer=outer,adj=adj,main="C",cex.main=cex,col="black",font=2,line=line)

Also, if you want the labels to be in the side, you can use mtext instead title:

line = 6
cex = 2
las = 2

par(mfrow=c(3,1), oma=c(1,6,1,1))

plot(rnorm(100),col="red")
mtext("A", side=2, line=line, cex=cex, las=las)
plot(rnorm(100),col="blue")
mtext("B", side=2, line=line, cex=cex, las=las)
plot(rnorm(100),col="green")
mtext("C", side=2, line=line, cex=cex, las=las)

Another option, to have the labels in the corner, is:

line = 1
cex = 2
side = 3
adj=-0.05

par(mfrow=c(3,1), oma=c(1,6,1,1))

plot(rnorm(100),col="red")
mtext("A", side=side, line=line, cex=cex, adj=adj)
plot(rnorm(100),col="blue")
mtext("B", side=side, line=line, cex=cex, adj=adj)
plot(rnorm(100),col="green")
mtext("C", side=side, line=line, cex=cex, adj=adj)

It is possible to use negative values for adj.