import numpy as np
import matplotlib.pyplot as plt
from matplotlib.figure import figaspect

f = lambda x: 1 / (1 + np.exp(-x))
xlim = (-3, 3)
x = np.linspace(xlim[0], xlim[1], 100)

fig = plt.figure(figsize=figaspect(1))
ax = fig.gca()
ax.plot(x, f(x), color='b')
ax.hlines((0, 1), xlim[0], xlim[1], color='k', alpha=0.3, linestyle='dashed')
ylim = ax.get_ylim()
ax.vlines(0, ylim[0], ylim[1], color='k', alpha=0.3, linestyle='dashed')
ax.set(xlim=xlim, ylim=ylim, xticks=(0, ), yticks=(0, 1), xlabel='$x$')
ax.get_yaxis().get_label().set(
    text=r'$\frac{1}{1+e^{-x}}$',
    size=15,
    rotation=0,
    horizontalalignment='right')
plt.show()