def to_xml_string(str = '')
      super(str) do
        str << '<c:pie3DChart>'
        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
        @series.each { |ser| ser.to_xml_string(str) }
        d_lbls.to_xml_string(str) if @d_lbls
        str << '</c:pie3DChart>'
      end
    end
def to_xml_string(str = '')
      super(str) do
        str << '<c:pie3DChart>'
        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
        @series.each { |ser| ser.to_xml_string(str) }
        d_lbls.to_xml_string(str) if @d_lbls
        str << '</c:pie3DChart>'
      end
    end
Creates a new pie chart object
 @param [GraphicFrame] frame The workbook that owns this chart.
 @option options [Cell, String] title
 @option options [Boolean] show_legend
 @option options [Symbol] grouping
 @option options [String] gap_depth
 @option options [Integer] rot_x
 @option options [String] h_percent
 @option options [Integer] rot_y
 @option options [String] depth_percent
 @option options [Boolean] r_ang_ax
 @option options [Integer] perspective
 @see Chart
 @see View3D
 Serializes the object
 @param [String] str
 @return [String]