ellipsefun <-
function(x, y, ...) {
# function for plotting ellipses to be used with xyplot()
# these are 95% confidence ellipses (adjust the level = argument
# omit center.pch argument to plot a symbol at the ellipse centroid
panel.xyplot(x, y, ...)
           panel.ellipse(x, y, level = .95, center.pch=NULL, ...)
       }