Add, remove, or leave alone the ``<c:legend>`` child element depending
        on current state and *bool_value*. If *bool_value* is |True| and no
        ``<c:legend>`` element is present, a new default element is added.
        When |False|, any existing legend element is removed.

def has_legend(self, bool_value):
        """
        Add, remove, or leave alone the ``<c:legend>`` child element depending
        on current state and *bool_value*. If *bool_value* is |True| and no
        ``<c:legend>`` element is present, a new default element is added.
        When |False|, any existing legend element is removed.
        """
        if bool(bool_value) is False:
            self._remove_legend()
        else:
            if self.legend is None:
                self._add_legend()