Allows a variety of shapes to be overlayed on the graph. The
shapes are identified by name and defined by position (X,Y)
and the parameters DATA1, DATA2, DATA3 (not all apply to all
shapes).
Shapes Defining quantities
VECTOR Offsets DATA1 from X and DATA2 from Y
CIRCLE Radius DATA1 from centre (X,Y)
ELLIPSE Centre (X,Y), Semi-major axis DATA1,
Ratio of axes major/minor DATA2 and
position angle DATA3
BOX Centre (X,Y) and lengths of sides
DATA1 and DATA2
All positions and dimensions are in axis units, angles are
in degrees. For circles and ellipses the axes need to be
equally scaled. This is normally the default for 2D data,
but can be forced using 'GSET AXES SCALED'.