Set the value of the ``<c:max>`` child element to the float *value*,
        or remove the max element if *value* is |None|.

def maximum(self, value):
        """
        Set the value of the ``<c:max>`` child element to the float *value*,
        or remove the max element if *value* is |None|.
        """
        self._remove_max()
        if value is None:
            return
        self._add_max(val=value)