## Dotplots a la Box, Hunter and Hunter

stripplot <- function(x, method="overplot", jitter=0.1, offset=1/3,
		      vertical=FALSE, group.names,
		      xlim=NULL, ylim=NULL, main="", ylab="", xlab="",
		      pch=0, col=par("fg"), cex=par("cex"))
{
    method <- pmatch(method, c("overplot", "jitter", "stack"))[1]
    if(is.na(method) || method==0)
	error("invalid plotting method")
    groups <-
	if(is.language(x)) {
	    if (inherits(x, "formula") && length(x) == 3) {
		groups <- eval(x[[3]], sys.frame(sys.parent()))
		x <- eval(x[[2]], sys.frame(sys.parent()))
		split(x, groups)
	    }
	}
	else if(is.list(x)) x
	else if(is.numeric(x)) list(x)
    if(0 == (n <- length(groups)))
	stop("invalid first argument")
    if(!missing(group.names))
	attr(groups, "names") <- group.names
    else if(is.null(attr(groups, "names")))
	attr(groups, "names") <- 1:n
    dlim <- rep(NA, 2)
    for(i in groups)
	dlim <- range(dlim, i[is.finite(i)], na.rm = TRUE)
    glim <- c(1, n)
    if(method == 2) { # jitter
	glim <- glim +	jitter * if(n == 1) c(-5, 5) else c(-2, 2)
    } else if(method == 3) { # stack
	glim <- glim + if(n == 1) c(-1,1) else c(0, 0.5)
    }
    if(is.null(xlim)) {
	xlim <- if(vertical) glim else dlim
    }
    if(is.null(ylim)) {
	ylim <- if(vertical) dlim else glim
    }
    plot(xlim, ylim, type="n", ann=FALSE, axes=FALSE)
    box()
    if(vertical) {
	if(n > 1) axis(1, at=1:n, lab=names(groups))
	axis(2)
    }
    else {
	axis(1)
	if(n > 1) axis(2, at=1:n, lab=names(groups))
    }

    csize <- cex*
	if(vertical) xinch(par("cin")[1]) else yinch(par("cin")[2])
    f <- function(x) seq(length=length(x))
    for(i in 1:n) {
	x <- groups[[i]]
	y <- rep(i,length(x))
	if(method == 2)
	    y <- y + runif(length(y), -jitter, jitter)
	else if(method == 3) {
	    xg <- split(x, factor(x))
	    xo <- lapply(xg, f)
	    x <- unlist(xg, use.names=FALSE)
	    y <- y + (unlist(xo, use.names=FALSE) - 1) * offset * csize
	}
	if(vertical) points(y, x, col=col[(i - 1)%%length(col) + 1],
			    pch=pch[(i - 1)%%length(pch) + 1], cex=cex)
	else points(x, y, col=col[(i - 1)%%length(col) + 1],
		    pch=pch[(i - 1)%%length(pch) + 1], cex=cex)
    }
    title(main=main, xlab=xlab, ylab=ylab)
}