#
# A wrapper around the function 'plot()' that jitters variables with
# 8 or less values, and provides more space around the pointclouds.
# [The implementation is a nasty hack: it adds to more extreme points to the data...]
#
# The number 8 can be changed.
# The jitter amount is .4 of the smallest spacing between
# different values.
# Again, the number .4 can be changed.
#
# One of the great annoyances in R's scatterplots is that the extreme
# points are too close to the borders (top and right) and axes (bottom and left).
# This function provides 10% of the range on both sides as extra space.
# The number 10% can be changed with the 'extra' argument.
#
# [Also, the plotting character default is changed to 16, and
#  plotting character size to .8.]
#
plot.plus <<- function(dat, max.ncat=8, jitter=.1, extra=.1,
                         lims=NULL, cex=0.8, pch=16, col="black", ...) {
  for(j in 1:ncol(dat))
    if(!(class(dat[,j]) %in% c("numeric","integer"))) dat[,j] <- c(factor(dat[,j]))
  # Jitter:
  for(i in 1:ncol(dat)) {
    x <- dat[,i];  xvals <- unique(x)
    if(length(xvals) < max.ncat)
      { space <- min(diff(sort(xvals)))
        dat[,i] <- x + runif(length(x), -space*jitter, space*jitter) }
  }
  # Add extra space around the points by adding two points that are not drawn:
  if(is.null(lims)) {
    mins <- apply(dat, 2, min);  maxs <- apply(dat, 2, max);  ranges <- maxs-mins
    dat <- rbind(mins-extra*ranges, maxs+extra*ranges, dat)  
  } else {
    dat <- rbind(lims[1], lims[2], dat)  
  }
  # Handle addition of two points in col, cex, pch:
  col <- c(rep(par()$bg,2), rep(col,len=nrow(dat)-2))
  pch <- c(rep(1,2), rep(pch,len=nrow(dat)-2))
  # Finally:
  plot(dat, cex=cex, pch=pch, col=col, ...)
}