Return the ``<c:bubbleSize>`` element for this series as unicode
        text. This element contains the bubble size values for all the
        data points in the chart.

def bubbleSize_xml(self):
        """
        Return the ``<c:bubbleSize>`` element for this series as unicode
        text. This element contains the bubble size values for all the
        data points in the chart.
        """
        return self._bubbleSize_tmpl.format(**{
            'nsdecls':    '',
            'numRef_xml': self.numRef_xml(
                self._series.bubble_sizes_ref, self._series.number_format,
                self._series.bubble_sizes
            ),
        })