code: def pt_xml(self, values):
          
          xml = (
              '                <c:ptCount val="{pt_count}"/>\n'
          ).format(
              pt_count=len(values)
          )
  
          pt_tmpl = (
              '                <c:pt idx="{idx}">\n'
              '                  <c:v>{value}</c:v>\n'
              '                </c:pt>\n'
          )
          for idx, value in enumerate(values):
              if value is None:
                  continue
              xml += pt_tmpl.format(idx=idx, value=value)
  
          return xml
docs: Return the ``<c:ptCount>`` and sequence of ``<c:pt>`` elements
          corresponding to *values* as a single unicode text string.
          `c:ptCount` refers to the number of `c:pt` elements in this sequence.
          The `idx` attribute value for `c:pt` elements locates the data point
          in the overall data point sequence of the chart and is started at
          *offset*.