## Plotting Curves Parallel to Axes

As there is no PlotPlot 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 astCurveastCurve and astPolyCurveastPolyCurve (§21.3) 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 astGridLineastGridLine function allows these curves to be drawn, as follows:

294#294

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.