This routine draws a general user-defined curve defined by the
supplied Mapping. Note that the curve is transformed into graphical
coordinate space for plotting, so that a straight line in
physical coordinates may result in a curved line being drawn if
the Mapping involved is non-linear. Any discontinuities in the
Mapping between physical and graphical coordinates are
catered for, as is any clipping established using AST_CLIP.
If you need to draw simple straight lines (geodesics), AST_CURVE
or AST_POLYCURVE will usually be easier to use and faster.