R/ellipse.default.R

In ellipse: Functions for Drawing Ellipses and Ellipse-Like Confidence Regions

"ellipse.default" <-
  function (x, scale = c(1, 1), centre = c(0, 0), level = 0.95,
            t = sqrt(qchisq(level, 2)), which = c(1, 2), npoints = 100,
            center = centre, ...)
{
  if(!missing(centre) && !missing(center)){
    warning("Specify centre or center, not both. The value from center will be used.")
  }

  names <- c("x", "y")
  if (is.matrix(x)) {
    xind <- which[1]
    yind <- which[2]
    r <- x[xind, yind]
    if (missing(scale)) {
      scale <- sqrt(c(x[xind, xind], x[yind, yind]))
      if (scale[1] > 0) r <- r/scale[1]
      if (scale[2] > 0) r <- r/scale[2]
    }
    if (!is.null(dimnames(x)[[1]])) 
      names <- dimnames(x)[[1]][c(xind, yind)]
  }
  else r <- x
  r <- min(max(r,-1),1)  # clamp to -1..1, in case of rounding errors
  d <- acos(r)
  a <- seq(0, 2 * pi, len = npoints)
  matrix(c(t * scale[1] * cos(a + d/2) + center[1], t * scale[2] * 
           cos(a - d/2) + center[2]), npoints, 2, dimnames = list(NULL, 
                                                    names))
}