[R] Plotting a legend outside the figure - units

[rhelp.20.trevva at spamgourmet.com]

Dear R gurus,

I am trying to plot a legend in the margins of a figure. The basic idea is to have two (or more) plots on the same figure, but then to have a common legend at the bottom of the plot. The approach that I have taken is to setup the figures, then do a "dummy" plot of the legend to determine the height of the legend box (ie from the value returned by the legend() command). I then use this height to adjust the outer margin of the plot to make sufficient space for the legend. After plotting the actual plots, I then finally plot the legend, moving it down to the bottom using the inset option in legend().

The problem is a question of units. I have not been able to find what units legend() returns anywhere in the documentation (ie in rect$h) - I've made the asumption that its inches, but that may not necessarily be correct. Similarly, the inset option to legend() requires units equal to a fraction of the plot size, but I'm not exactly sure how to make that line up with what I get out of legend().

Suggestions and insights most welcome! Code follows at the bottom.

Cheers,

Mark
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graphics.off()
#First set number of plots
par(mfrow=c(2,1))

#Generate the legend once to get the height
leg <-  legend("bottom",legend=c("Item A","Item B"),horiz=TRUE,pch=c(1,2),
          col=c("red","blue"),plot=FALSE,trace=TRUE)

par(oma=c(5,4,4,2)+0.1,mar=c(0,0,0,0),xpd=NA)

#Add space at the bottom for the legend
bottom.spacing  <-  par("omi") + c(leg$rect$h,0,0,0)
par(omi=bottom.spacing)

x <- runif(100)
y <- runif(100)
z <- rnorm(100)
plot(x,y,xaxt="n",xlim=c(0,1),xlab="",pch=1,col="red")
plot(y,z,xlim=c(0,1),pch=2,col="blue")

#Actually plot legend this time - inset value is done manually by trial
#and error, but obviosuly I'd rather have this done intelligently
legend("bottom",legend=c("Item A","Item B"),horiz=TRUE,pch=c(1,2),
    col=c("red","blue"),inset=c(0,-0.475))