def add_chart(chart_type, options)
      @drawing ||= Drawing.new worksheet
      drawing.add_chart(chart_type, options)
    end
def add_chart(chart_type, options)
      @drawing ||= Drawing.new worksheet
      drawing.add_chart(chart_type, options)
    end
adds a chart to the drawing object
 @param [Class] chart_type The type of chart to add
 @param [Hash] options Options to pass on to the drawing and chart
 @see Worksheet#add_chart