##########################################################################
#                                                                        #
# Elementary version of an R function for drawing a Location-Spread plot #
#                                                                        #
##########################################################################

location.spread.plot <-

  function(loc.in,    # vector of location values 
           sprd.in,   # vector of spread values
                      # loc.in and sprd.in must be of equal length
           ww = pretty(c(loc.in,sprd.in))[length(pretty(c(loc.in,sprd.in)))],
                      # nice plotting range
           n.l = 10,  # number of equally spaced, 45 degree reference rays
                      # to be drawn.  Set n.l <= 20 for best results
           col.plot = rep(1,length(loc.in)),  # vector of plotting colors
           pch = 16,  # plotting character
           xlab = "Location",  #  x-axis lable
           ylab = "Spread",    #  y-axis lable
           main = "Location-Spread Plot",  # main title
           pin = 3    # height and half-width of the plot in inches
           ){

# store the current values of the graphics parameters
    
    op <- par(pin=c(2*pin,pin))

# set up the axes and labels
    
    plot(loc.in,sprd.in,xlim=c(-ww,ww),ylim=c(0,ww),
         xlab=xlab,ylab=ylab,main=main,type="n")

# set up the colors for the 45 degree reference rays according to a heat
# color map    
    
    c.offset <- floor(n.l/4)
    col.l <- heat.colors(n.l+2*c.offset)[(n.l+c.offset):(c.offset+1)]

# Make the background color of the quadrant delimited by the two main
# diagonal through the origin a pale yellow    
    
    a <- 0
    b <- 0
    d <- ww/(n.l-1)

    x <- c(a,a+ww,a-ww,a)
    y <- c(b,b+ww,b+ww,b)
    polygon(x,y, xpd=FALSE, col = heat.colors(30)[29],
            lty=2, lwd=2, border = NA)

# Draw the 45 degree refence rays
    
    for(j in seq(1,n.l)){
      x <- c(a+(j-1)*d,ww)
      y <- c(0,ww-(j-1)*d)
      lines(x,y,lty=1,col=col.l[j])
      x <- c(a-(j-1)*d,-ww)
      y <- c(0,ww-(j-1)*d)
      lines(x,y,lty=1,col=col.l[j])
    }

# Draw a vertical reference ray through the origin
    
    lines(c(0,0),c(0,ww),col=col.l[n.l])

# Plot the location-spread points
    
    points(loc.in,sprd.in,pch=pch,col=col.plot)

# Restore the original values of the graphics parameters
    
    par(op)
    
  }