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