As there is no Plot routine to draw a ``straight line'', drawing axes and grid lines to represent coordinate systems requires a slightly different approach. The problem is that for some coordinate systems, these grid lines will not be geodesics, so AST_CURVE and AST_POLYCURVE () cannot easily be used (you would have to resort to approximating grid lines by many small elements). Lines of constant celestial latitude provide an example of this, with the exception of the equator which is a geodesic.
The AST_GRIDLINE routine allows these curves to be drawn, as follows:
INTEGER AXIS DOUBLE PRECISION LENGTH ... CALL AST_GRIDLINE( PLOT, AXIS, START, LENGTH, STATUS )
Here, AXIS specifies which physical coordinate axis we wish to draw
parallel to. The START array contains the coordinates of the start of
the curve and LENGTH specifies the distance to draw along the axis in
physical coordinate space.
AST A Library for Handling World Coordinate Systems in Astronomy