def plot_steady_states(ax, states, offset=0, 
                       color='g', label='Steady state'):
    """
    Args:
      * ax (Axes)
      * states (pd.DataFrame): Steady States
      * offset (int or float): add this to ss.mean
      * color (str)
      * label (str)

    Returns:
      * line
    """
    line = None
    for start, val in states.iterrows():
        end = val['end']
        try:
            mean = val['power'].mean + offset
        except AttributeError:
            mean = val['power'].get_model().mean + offset
        line, = ax.plot([start, end], [mean, mean], color=color, 
                     linewidth=2, alpha=0.6)
    if line is not None:
        line.set_label(label)
    return line