def draw_line(vis, step, lines, names):
    # draw multiple lines in one panel.
    """

    :param vis: the object of the visdom.Visdom
    :param step: the step of the line
    :param lines: the lines tuple, (line1, line2, ...)
    :param names: the names tuple, (name1, name2, ...)
    :return: None
    """
    if not len(lines) == len(names):
        raise ValueError('The length of the input is not the same')

    win_name = ''
    for i in range(len(names)):
        win_name += names[i]

    if step == 0:
        for line, name in zip(lines, names):
            vis.line(X=torch.Tensor([step]),
                     Y=torch.Tensor([line]),
                     win=win_name,
                     name='%s' % name,
                     opts=dict(legend=[name])
                     )
    else:
        for line, name in zip(lines, names):
            vis.updateTrace(X=torch.Tensor([step]),
                            Y=torch.Tensor([line]),
                            win=win_name,
                            name='%s' % name
                            )