Return new `p:graphicFrame` element appended to this shape tree.

        The `p:graphicFrame` element has the specified position and size and
        refers to the chart part identified by *rId*.

def _add_chart_graphicFrame(self, rId, x, y, cx, cy):
        """Return new `p:graphicFrame` element appended to this shape tree.

        The `p:graphicFrame` element has the specified position and size and
        refers to the chart part identified by *rId*.
        """
        shape_id = self._next_shape_id
        name = 'Chart %d' % (shape_id-1)
        graphicFrame = CT_GraphicalObjectFrame.new_chart_graphicFrame(
            shape_id, name, rId, x, y, cx, cy
        )
        self._spTree.append(graphicFrame)
        return graphicFrame