Add confidence ellipse or convex hull for group of points
plotPointsShape(p, lwd, lty, opacity, shape_function, ...)
plot data returned by function `mdaplot()`
thickness of line used to show the hull
type of line used to show the hull
of opacity is larger than 0 a semi-transparent polygon is shown over points
function which calculates and return coordinates of the shape
extra parameters for shape_function