def argmin(self):
        """
        Return the co-ordinates of the bin centre containing the
        minimum value.  Same as numpy.argmin(), converting the
        indexes to bin co-ordinates.
        """
        return tuple(centres[index] for centres, index in
                     zip(self.centres(), numpy.unravel_index(self.array.argmin(),
                                                             self.array.shape)))