Return the rId of a new |ChartPart| object containing a chart of
        *chart_type*, displaying *chart_data*, and related to the slide
        contained in this part.

def add_chart_part(self, chart_type, chart_data):
        """
        Return the rId of a new |ChartPart| object containing a chart of
        *chart_type*, displaying *chart_data*, and related to the slide
        contained in this part.
        """
        chart_part = ChartPart.new(chart_type, chart_data, self.package)
        rId = self.relate_to(chart_part, RT.CHART)
        return rId