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

def minimum(self, value):
        """
        Set the value of the ``<c:min>`` child element to the float *value*,
        or remove the min element if *value* is |None|.
        """
        self._remove_min()
        if value is None:
            return
        self._add_min(val=value)