If the Invisible value of a Plot is non-zero, then all the Plot methods which normally generate graphical output do not do so (you can think of them drawing with "invisible ink"). Such methods do, however, continue to do all the calculations which would be needed to produce the graphics. In particular, the bounding box enclosing the graphics is still calculated and can be retrieved as normal using AST_BOUNDINGBOX. The default value is zero, resulting in all methods drawing graphics as normal, using visible ink.
AST A Library for Handling World Coordinate Systems in Astronomy