import numpy as np
from matplotlib import pyplot as plt

plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

x = np.array([3, 4, 5, 6, 7, 8, 9])
y = np.array([6, 5, 4, 3, 2, 1, 6])

fig, ax = plt.subplots()
ax.plot(x, y, 'o-', label='线性图')
y_avg = [np.mean(x)] * len(x)
ax.plot(x, y_avg, color='red', lw=6, ls='--', label="平均线性图")

plt.legend(loc=0)
plt.show()