如何在matplotlib中给已经画好的线添加一条线？

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

x1 = [1, 2, 3, 4, 5]
y1 = [2, 4, 6, 8, 10]
line1, = ax.plot(x1, y1, label='Line 1')

x2 = [1, 2, 3, 4, 5]
y2 = [5, 4, 3, 2, 1]
line2, = ax.plot(x2, y2, label='Line 2')

x3 = [1, 2, 3, 4, 5]
y3 = [3, 3, 3, 3, 3]
line3, = ax.plot(x3, y3, label='Line 3')

ax.legend()

plt.show()