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?

Pitzer

In addition to the solutions listed below, for stable results, you should use another device rather than messing around with your IDE's preview window and trying to export graphics from there. – Matte

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")

Anaximander

Thanks again for you answer. I seem to be running in to trouble with this solution as it cuts off the edge of the graph (see right hand of graph to the right in update). If I 'squash' the graph the point becomes visible. I am using the same setting as you have shown above. Any idea what may be going on? – Pitzer

I presume you have already looked at lattice or ggplot to achieve the same effect with facets? That's what I would do, instead of faffing about with manual placement of legends. – Anaximander

Hmm..no I am just using layout() as suggested above. Should I abandon the approach above and use lattice instead? – Pitzer

Probably. Or ggplot. (IMHO). But it depends on your problem. These packages are great for data with different facets. clearly if you want to create completely different plots with a common legend you'll run into the same problem. – Anaximander

x- and y-labels seem to not show when plotting like this, for some reason. – Catchpole

Answer 2

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")

Ergener

Thank you. Always good to see alternative ways to go about things. – Pitzer

If you are going to use this approach then you should also learn the grconvertX and grconvertY functions for finding the coordinates to place the legend. – Highams

It is wizard. I run this code but I don't get the same output like you. – Stupid