Every Plot is also a Frame, so can be used to obtain the values of Frame attributes such as a Title, axis Labels, axis Units, etc., which are typically used when displaying data and/or coordinates. These attributes are, as for any FrameSet, derived from the current Frame of the Plot (). They are also used automatically when using the Plot to plot coordinate axes and coordinate grids (e.g. for labelling them--).
Because the current Frame of a Plot represents physical coordinates,
any Frame operation applied to the Plot will effectively be working in
this coordinate system. For example, the AST_DISTANCE and AST_OFFSET
routines will compute distances and offsets in physical coordinate
space, and AST_FORMAT will format physical coordinates in an
appropriate way for display.
AST A Library for Handling World Coordinate Systems in Astronomy