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

par(xpd=NA) is more what you're looking for. Extract from the ?par help page:

xpd
A logical value or NA. If FALSE, all plotting is clipped to the plot region, if TRUE, all plotting is clipped to the figure region, and if NA, all plotting is clipped to the device region. See also clip.

Indeed, you want it to be clipped to the device region and not the figure region (see, for instance, this blog entry for a graphical explanation of the differences between the plot, the figure and the device regions).

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

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

plot(1:3,4:6,main="plot 2")
legend(-0.5,3.5,ncol=3,c("0-1 km","1-5 km","outside barrier"), 
    fill=c("green","orange","red"), title="Fetch")

Answer 2

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")