code: def add_chart_part(self, chart_type, chart_data):
          
          chart_part = ChartPart.new(chart_type, chart_data, self.package)
          rId = self.relate_to(chart_part, RT.CHART)
          return rId
docs: 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.