>>> from scipy.sparse import coo_matrix
>>> sigma_x = np.array([[0, 1], [1, 0]])
>>> signal = graph.pwc_signal(values=np.array([1, 2, 3]), duration=0.1)
>>> graph.sparse_pwc_operator(signal=signal, operator=coo_matrix(sigma_x))
<SparsePwc: operation_name="sparse_pwc_operator", value_shape=(2, 2)>