This attribute selects the functions which are used to draw graphics by
the Plot class. If it is zero, then the functions in the graphics
interface selected at link-time are used (see the ast_link script).
Otherwise, functions registered using astGrfSet are used. In this
case, if a function is needed which has not been registered,
then the function in the graphics interface selected at link-time is
used.
The default is to use the graphics interface