R help - Jul 2003 - Positioning legends outside the plot region

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

Marco Albani wrote:
> 
> 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?
> 
I answer my own question:


## 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])

## calculate the ratio of inches to y user coords
in2y <- (par("usr")[4] -
par("usr")[3])/par("pin")[2]

## place y at the bottom of the margin
y <-  par("usr")[3] - par("mai")[1] * in2y

## justify the legend so that is centered in x (xjust=0.5)
## and on top of y (yjust=0)
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, yjust=0
      )

par(old.par)


-- 
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