如何在python bode图中绘制边距？

import numpy as np
import matplotlib.pyplot as plt
from scipy import signal

# 定义系统的传递函数
num = [1]
den = [1, 1, 1]

# 计算系统的频率响应
sys = signal.TransferFunction(num, den)
w, mag, phase = signal.bode(sys)

# 绘制幅频特性曲线
plt.subplot(2, 1, 1)
plt.semilogx(w, mag)
plt.xlabel('Frequency [rad/s]')
plt.ylabel('Magnitude [dB]')
plt.grid(True)

# 绘制相频特性曲线
plt.subplot(2, 1, 2)
plt.semilogx(w, phase)
plt.xlabel('Frequency [rad/s]')
plt.ylabel('Phase [degrees]')
plt.grid(True)

# 标注边距
gm, pm, wg, wp = signal.margin(sys)
plt.subplot(2, 1, 1)
plt.plot(wg, gm, 'ro', label='Gain Margin')
plt.subplot(2, 1, 2)
plt.plot(wp, -pm, 'ro', label='Phase Margin')

# 显示图例
plt.subplot(2, 1, 1)
plt.legend()
plt.subplot(2, 1, 2)
plt.legend()

# 显示图形
plt.show()