[R] Positioning legends outside the plot region

[Marco Albani]

Hello,

I understand this issue has already been discussed
http://maths.newcastle.edu.au/~rking/R/help/01c/3763.html
but there is one point I am trying to understand better.

I want to lace a legend below the plotting region.
Right now I do


## increase size of the right hand border (secondary axis)
## and bottom border (legend)
old.mar <- par("mar")
old.mar[1] <- 7.1
old.mar[4] <- 7.1
old.par <- par(mar=old.mar)

plot(data$t, data$w, type="l",
        xlab =x.lab, ylab = y.lab, col="black", lwd=2,
        ...)

   ## Plot the fluxes on a secondary axis

    x <- range(data$w)
    y <- range(c(data$rain,data$uptake,data$perc))
    K <-  x[1] - y[1]*(x[2]-x[1])/(y[2]-y[1])
    R <- (x[2]-x[1])/(y[2]-y[1])
    y.seq <- pretty(y)

  lines(data$t, (K + R * data$uptake) , col ="green")
   axis(4, at = (K + R * y.seq), labels = y.seq)
   mtext(y2.lab, side=4, line=2)

   title(main = paste("Soil Water dynamics for site\n",data$site[1]),
       sub = NULL, xlab = NULL, ylab = NULL,
       line = NA, outer = FALSE)

  ## Set xpd=TRUE, so all plotting is clipped to the figure
  ## (not the plot) region:
  old.par <- par(xpd=TRUE)

## add the legend outside the plot on the bottom,
## centered on the plot x axis

x <- mean(par("usr")[1:2])
y <- par("usr")[3] - 1

     legend(x,y,
            legend=c("Soil Water", "Precipitation","Percolation","Uptake"),
            col=c("black","blue","red","green"),
            lwd=c(2,1,1,1),
            bty="n", xjust=0.5, ncol=2,
      )

  par(old.par)


The part I am not happy with is the setting of the y coordinates for the 
legend.

Is there a good way to tie it in with the par("mar")[1] so that I know 
the legend is place below the title of the x axis of my plot?




-- 
Marco Albani, Ph.D.
Postdoctoral Fellow
Dept. of Organismic and Evolutionary Biology
Harvard University
HUH 22 Divinity Avenue
Cambridge, MA
02138-2094 USA

Tel: +1 617 495 1621
Fax: +1 617 495 9484

http://www.people.fas.harvard.edu/~malbani