def histogram(self, bmus=None):
        """\
        Return a 2D histogram of bmus.

        :param bmus: the best-match units indexes for underlying data.
        :type bmus: :class:`numpy.ndarray`
        :returns: the computed 2D histogram of bmus.
        :rtype: :class:`numpy.ndarray`
        """
        if bmus is None:
            assert self._bmus is not None, 'not trained'
            bmus = self._bmus
        arr = np.zeros((self._som.nrows, self._som.ncols))
        for i,j in bmus:
            arr[i,j] += 1
        return arr