This function joins a series of points specified in the physical
coordinate system of a Plot by drawing a sequence of geodesic
curves. It is equivalent to making repeated use of the astCurve
function (q.v.), except that astPolyCurve will generally be more
efficient when drawing many geodesic curves end-to-end. A
typical application of this might be in drawing contour lines.
As with astCurve, full account is taken of the Mapping between
physical and graphical coordinate systems. This includes any
discontinuities and clipping established using astClip.