python matplotlib 画一条水平直线遇到的问题

import numpy as np                 #使用import导入模块numpy，并简写成np
import matplotlib.pyplot as plt    #使用import导入模块matplotlib.pyplot，并简写成plt
plt.figure(figsize=(8,4))          #设置绘图对象的宽度和高度

t = np.arange(0,1.1,0.1)
theta = 30+15*(t**2) 
theta_v = 30*t 
theta_accel = 30  

plt.plot(t,theta,label="$theta$",color="red",linewidth=2)
plt.plot(t,theta_v,label="$thetaV$",color="green",linewidth=2)  
#plt.plot(t,theta_accel,label="$thetaAccel$",color="blue",linewidth=2)

t = np.arange(1,3.1,0.1)
theta = 45+30*(t-1) 
theta_v = 30 
theta_accel = 0  

plt.plot(t,theta,label="$theta$",color="red",linewidth=2)
#plt.plot(t,theta_v,label="$thetaV$",color="green",linewidth=2)  
#plt.plot(t,theta_accel,label="$thetaAccel$",color="blue",linewidth=2)

t = np.arange(3,4.1,0.1)
theta = 120-15*((4-t)**2)
theta_v = 30*(4-t) 
theta_accel = -30  

plt.plot(t,theta,label="$theta$",color="red",linewidth=2)
plt.plot(t,theta_v,label="$thetaV$",color="green",linewidth=2)  
#plt.plot(t,theta_accel,label="$thetaAccel$",color="blue")

plt.ylim(-40,200)             #使用plt.ylim设置y坐标轴范围
plt.xlim(-1,5)
plt.xlabel("Time(s)")         #用plt.xlabel设置x坐标轴名称
plt.legend(loc='upper left')  #设置图例位置
plt.grid(True)
plt.show()
import nu