The keyword '_nologend_' prevents a plot to show up in the legend of matplotlib figure

import numpy as np 
import matplotlib.pyplot as plt 

x1,y1 = np.random.rand(2,10) 
x2,y2 = np.random.rand(2,10) 
x3,y3 = np.random.rand(2,10) 
x4,y4 = np.random.rand(2,10) 

plt.plot(x1, y1, label='Test 1') 
plt.plot(x2, y2) #No label, so  no legend text 
plt.plot(x3, y3, label='_nolegend_') # Activ no legend text
plt.plot(x4, y4, label='Test 4') 

plt.legend()  # legend with entries from the first and fourth plot