As there is no Plot function 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 astCurve and astPolyCurve
() 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 astGridLine function allows these curves to be drawn, as follows:
int axis; double length; ... astGridLine( plot, axis, start, length );
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