Always add a ``<c:autoUpdate val="0"/>`` child so auto-updating
        behavior is off by default.

def _add_externalData(self):
        """
        Always add a ``<c:autoUpdate val="0"/>`` child so auto-updating
        behavior is off by default.
        """
        externalData = self._new_externalData()
        externalData._add_autoUpdate(val=False)
        self._insert_externalData(externalData)
        return externalData