#Imports===================================
import matplotlib.pyplot as plt
import numpy as np
#Functions=================================
def plot_sigmoid(z):
    """ TODO """
    return 1.0 / (1.0 + np.exp(-z))
#Program===================================
z = np.arange(-7, 7, 0.1)
phi_z = plot_sigmoid(z)
plt.plot(z, phi_z)
plt.axvline(0.0, color='k')
plt.axhspan(0.0, 1.0, facecolor='1.0', alpha=1.0, ls='dotted')
plt.axhline(y=0.5, ls='dotted', color='k')
plt.axhline(y=0, ls='dotted', color='k')
plt.axhline(y=1, ls='dotted', color='k')
plt.yticks([0.0, 0.5, 1.0])
plt.ylim(-0.1, 1.1)
plt.xlabel('z')
plt.ylabel(r'$\phi (z)$', rotation='horizontal', labelpad=15)
plt.show()