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*.

def pt_xml(self, values):
        """
        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*.
        """
        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