title: draw a circle in ggplot2 that encompases 90% of points
body: <p>I have a very specific problem: basically I have 2000 points on a plot that are stratified in 4 groups of unequal size. I need to illustrate group, BUT I cannot use colour or shape of the points. Instead I want to draw an oval that encompases say 90% of the data. Is there a function for this?</p>