I need to know where the Sprite.graphics start to do some stuff. When I add the sprite to my MovieClip, (x,y) coordinates are (0,0) but I need to know coordinates from Sprite.graphics, not Sprite. There are some images so clarify what I want:

I have this board where user can draw some forms (lines, arrows, polygons, etc.) When the user draw a line, I interpret that Sprite is created (with Sprite.graphics inside it, the real form).

How can I catch the point where Sprite.graphics is created? Actually I can’t because Sprite.graphics is read-only.
Thanks!
EDIT: some source code
line = new Sprite();
line.graphics.moveTo(posX,posY);
line.graphics.lineStyle(10);
line.graphics.lineTo(endposX,endposY);
Global.board.addChild(line);
To get the bounds of visual part, you can try:
There is also a getRect() method, but that leaves out space taken by strokes.