import numpy as np
from matplotlib import pyplot as plt
from labellines import labelLines

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

X = np.linspace(0, 1, 500)
A = [1, 2, 5, 10, 20]

for a in A:
   plt.plot(X, np.arctan(a*X), label=str(a))

labelLines(plt.gca().get_lines(), zorder=2.5)

plt.show()