Centring legend below two plots in r

Question

I would like to centre a common legend below two plots. I have used xpd=TRUE to allow for printing outside the plot itself and oma to create space for the legend. However the legend will not move horizonatally and gets clipped 'early' vertically. Any advice?

quartz(title="PCoA",12,6)
par(mfrow=c(1,2),oma=c(5,0,0,0),xpd=TRUE)

plot(1:3,4:6,main="plot 1")

plot(1:3,4:6,main="plot 2")


# Clips the plot    
legend(1,3.5,ncol=3,c("0-1 km","1-5 km","outside barrier"),fill=c("green","orange","red"), title="Fetch")

# Won't let me move the legend sideways 
legend(0,3.5,ncol=3,c("0-1 km","1-5 km","outside barrier"),fill=c("green","orange","red"), title="Fetch")

UPDATE

With the solution below it cuts of the edge of the graph which becomes visible by changing the dimension of the figure by dragging the edge (see below). Any ideas what might be going on?

Answer 1

Rather than using par=mfrow(...) I suggest you use layout().

This allows you to specify a matrix with plot positions:

layout(matrix(c(1,2,3,3), ncol=2, byrow=TRUE), heights=c(4, 1))

par(mai=rep(0.5, 4))
plot(1:3,4:6,main="plot 1")
plot(1:3,4:6,main="plot 2")

par(mai=c(0,0,0,0))
plot.new()
legend(x="center", ncol=3,legend=c("0-1 km","1-5 km","outside barrier"),
       fill=c("green","orange","red"), title="Fetch")