docs: function parse_drawing(data, rels) {
  	if(!data) return "??";
  	/*
  	  Chartsheet Drawing:
  	   - 20.5.2.35 wsDr CT_Drawing
  	    - 20.5.2.1  absoluteAnchor CT_AbsoluteAnchor
  	     - 20.5.2.16 graphicFrame CT_GraphicalObjectFrame
  	      - 20.1.2.2.16 graphic CT_GraphicalObject
  	       - 20.1.2.2.17 graphicData CT_GraphicalObjectData
            - chart reference
  	   the actual type is based on the URI of the graphicData
  		TODO: handle embedded charts and other types of graphics
  	*/
  	var id = (data.match(/<c:chart [^>]*r:id="([^"]*)"/)||["",""])[1];
  
  	return rels['!id'][id].Target;
  }