All Plots are also Mappings (just like the FrameSets from which they are derived), so can be used to transform coordinates.
Like FrameSets, the forward transformation of a Plot will convert coordinates between the base and current Frames (i.e. between graphical and physical coordinates). This would be useful if you were (say) reading a cursor position in graphical coordinates and needed to convert this into physical coordinates for display.
Conversely, a Plot's inverse transformation converts between its current and base Frames (i.e. from physical coordinates to graphical coordinates). This transformation is applied automatically whenever plotting operations are carried out by AST functions. It may also be useful to apply it directly, however, if you wish to perform additional plotting operations (e.g. those provided by the native graphics system) at positions specified in physical coordinates.
There is, however, one important difference between using a FrameSet
and a Plot to transform coordinates, and this is that clipping may be
applied by a Plot (if it has been enabled using
astClip--). Any point which lies within the
clipped region of a Plot will, when transformed, yield coordinates
with the value AST__BAD. If you wish to avoid this clipping, you
should extract the relevant Mapping from the Plot (using
astGetMapping) and use this, instead of the Plot, to transform the
AST A Library for Handling World Coordinate Systems in Astronomy