Return the values of the labels for the header of columns or rows.

        The value corresponds to the header of column or row x in the
        given level.

def header(self, axis, x, level=0):
        """
        Return the values of the labels for the header of columns or rows.

        The value corresponds to the header of column or row x in the
        given level.
        """
        ax = self._axis(axis)
        return ax.values[x] if not hasattr(ax, 'levels') \
            else ax.values[x][level]