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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)))
}
# need to add options for dealing with fewer than 2 points, more than 2 points, and NA values.
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