/* 20.5 DrawingML - SpreadsheetML Drawing */
function parse_drawing(data, rels/*:any*/) {
	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;
}