def expand_rmf_matrix(rmf):
    """Expand an RMF matrix stored in compressed form.

    *rmf*
      An RMF object as might be returned by ``sherpa.astro.ui.get_rmf()``.
    Returns:
      A non-sparse RMF matrix.

    The Response Matrix Function (RMF) of an X-ray telescope like Chandra can
    be stored in a sparse format as defined in `OGIP Calibration Memo
    CAL/GEN/92-002
    <https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html>`_.
    For visualization and analysis purposes, it can be useful to de-sparsify
    the matrices stored in this way. This function does that, returning a
    two-dimensional Numpy array.

    """
    n_chan = rmf.e_min.size
    n_energy = rmf.n_grp.size

    expanded = np.zeros((n_energy, n_chan))
    mtx_ofs = 0
    grp_ofs = 0

    for i in range(n_energy):
        for j in range(rmf.n_grp[i]):
            f = rmf.f_chan[grp_ofs]
            n = rmf.n_chan[grp_ofs]
            expanded[i,f:f+n] = rmf.matrix[mtx_ofs:mtx_ofs+n]
            mtx_ofs += n
            grp_ofs += 1

    return expanded