updateusr method for better barplot with lines/points

When using barplot with add y2 axis for lines or points by par(new=TRUE)  as show in the example code usually get shifted.

The updateusr method from Greg Snow's TeachingDemos package is a great solution for this problem. The idea behind updateusr is define new plot region according to the ratio of one unit in each axis.

The code:

	updateusr <- function(x1, y1=NULL, x2, y2=NULL) {
	xy1 <- xy.coords(x1,y1)
	xy2 <- if( missing(x2) && missing(y2) ) {
	xy.coords(y1)
	} else {
	xy.coords(x2,y2)
	}
	cur.usr <- par('usr')
	xslope <- diff(xy2$x)/diff(xy1$x)
	yslope <- diff(xy2$y)/diff(xy1$y)
	new.usr.x <- xslope * ( cur.usr[1:2] - xy1$x ) + xy2$x
	new.usr.y <- yslope * ( cur.usr[3:4] - xy1$y ) + xy2$y
	invisible(par(usr=c(new.usr.x, new.usr.y)))
	}

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Example code:(add space 8/20)

	yy <- c(4,6,13,24,15,13,14,8,2,1)
	n <- length(yy)
	bar <- data.frame(Group=LETTERS[1:n], value=yy*1000, yield=yy+rnorm(n,50,10))
	ylim <- c(0,max(bar$value))
	space <- 0.3
	xlim <- c(0, floor((1+space)*n)+1)
	plot(0, xaxs = "i", xlim = xlim, ylim = ylim, main = "Barplot with Line use par(new=TRUE)", xlab = "", ylab = "", type = "n", axes = FALSE)
	r <- barplot(bar$value, axes=F, axisnames = FALSE, space = space, add = T)
	axis(1, at = r, labels = LETTERS[1:n])
	axis(2, at = seq(0,ylim[2], 1000), las = 2)
	#use par new
	par(new = TRUE)
	plot(x = r, y = bar$yield, axes = FALSE, xlim = xlim, ylim = c(0,110) ,xlab = "", ylab = "",type = "o")
	abline(v=r, lty = 2, col = "red")
	axis(4, at = seq(0, 110, 10))
	box()
	plot(0, xaxs = "i", xlim = xlim, ylim = ylim, main = "Barplot with Line use updateusr", xlab = "", ylab = "", type = "n", axes = FALSE)
	r <- barplot(bar$value, axes=F, axisnames = FALSE, space = space, add = T)
	axis(1, at = r, labels = LETTERS[1:n])
	axis(2, at = seq(0,ylim[2], 1000), las = 2)
	#By using updateusr
	updateusr(r[1:2], ylim/100, r[1:2], 0:1)
	lines(x = r, y = bar$yield, type = "o")
	abline(v=r, lty = 2, col = "red")
	axis(4, at = seq(0, 110, 10))
	box()

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Result: