import matplotlib.pyplot as plt
import numpy as np
import math

def func1(x):
    return math.cos(x) - 3*math.sin(x)

def func2(x):
    if x>= math.pi and x<= 3*math.pi/2:
        tmp = math.cos(x) - 3*math.sin(x)

        if tmp < 0:
            return 0.5
        else:
            return tmp

data_x1 = [i for i in np.arange(0, math.pi, (2*math.pi/36))]
data_y1 = [func1(x) for x in data_x1]
data_x2 = [i for i in np.arange(math.pi, 3*math.pi/2 + 2*math.pi/36, (2*math.pi/36))]
data_y2 = [func2(x) for x in data_x2]
data_x3 = [i for i in np.arange(3*math.pi/2 + 2*math.pi/36, 2*math.pi, (2*math.pi/36))]
data_y3 = [func1(x) for x in data_x3]

plt.title('График cos(x) - 3*sin(x)')
plt.xlabel('x')
plt.ylabel('y')

plt.plot(data_x1, data_y1, 'r')
plt.plot(data_x2, data_y2, 'r')
plt.plot(data_x3, data_y3, 'r')

plt.grid(True, linestyle='-', color='black')

data_x1.extend(data_x2)
data_x1.extend(data_x3)
data_y1.extend(data_y2)
data_y1.extend(data_y3)
plt.show()