R-beta: plot with error bars?

[Bill Venables]

Bill Simpson writes:
 > I have read the Venables & Smith Notes on R.
 > I was wondering if someone could tell me how to make an XY plot with error
 > bars.
 > 1. x holds x values, y holds y values, ci contains confidence interval
 > (error bar stretches from y-ci to y+ci)
 > 2. x, y, ucl contains upper confidence limit, lcl contains lower
 > confidence limit.  Error bar stretches from lcl to ucl. Need this for
 > asymmetrical error bars.

Peter Dalgaard has already offered a suggestion but here is a
more formal function that I often find useful. 

plotCI <- function (x, y = NULL, uiw, liw = uiw, ..., sfrac = 0.01)  {
  if (is.list(x)) {
    y <- x$y
    x <- x$x
  }
  if (is.null(y)) {
    if (is.null(x)) 
      stop("both x and y NULL")
    y <- as.numeric(x)
    x <- seq(along = x)
  }
  ui <- y + uiw
  li <- y - liw
  plot(x, y, ylim = range(c(y, ui, li)), ...)
  smidge <- diff(par("usr")[1:2]) * sfrac
  segments(x, li, x, ui)
  x2 <- c(x, x)
  ul <- c(li, ui)
  segments(x2 - smidge, ul, x2 + smidge, ul)
  invisible(list(x = x, y = y))
}

The arguments are pretty self-explanatory, but here is a brief
run-down:

x, y are as you would expect, although both may be replaced by an
     xy-list, or if y is omitted, x is plotted against 1:length(x),

uiw is the distance(s) from y to the upper end of the interval,
liw is the distance(s) from y to the lower end of the interval,
     (if omitted taken to be the same as uiw),

... allows extra arguments to be passed to plot, eg xlab, ylab,
    type, lty, lwd, &c. (but not passed to segments()),

sfrac is the fraction of the x-axis width to be used as the
     half-length of the seriphs that customarily go at each end
     of the intervals, (by default 1%).

-- 
Bill Venables, Head, Dept of Statistics,    Tel.: +61 8 8303 5418
University of Adelaide,                     Fax.: +61 8 8303 3696
South AUSTRALIA.     5005.   Email: Bill.Venables at adelaide.edu.au
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