Save the current graphics functions used by a Plot
The AST_GRFPUSH and AST_GRFPOP functions are intended for situations where it is necessary to make temporary changes to the graphics functions used by the Plot. The current functions should first be saved by calling AST_GRFPUSH. New functions should then be registered using AST_GRFSET. The required graphics should then be produced. Finally, AST_GRFPOP should be called to restore the original graphics functions.