`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)))
}