Infinite lines

axvline and axhline draw infinite vertical / horizontal lines, at given x / y positions. They are usually used to mark special data values, e.g. in this example the center and limit values of the sigmoid function.

axline draws infinite straight lines in arbitrary directions.

import numpy as np
import matplotlib.pyplot as plt

t = np.linspace(-10, 10, 100)
sig = 1 / (1 + np.exp(-t))

plt.axhline(y=0, color="black", linestyle="--")
plt.axhline(y=0.5, color="black", linestyle=":")
plt.axhline(y=1.0, color="black", linestyle="--")
plt.axvline(color="grey")
plt.axline((0, 0.5), slope=0.25, color="black", linestyle=(0, (5, 5)))
plt.plot(t, sig, linewidth=2, label=r"$\sigma(t) = \frac{1}{1 + e^{-t}}$")
plt.xlim(-10, 10)
plt.xlabel("t")
plt.legend(fontsize=14)
plt.show()

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References

The use of the following functions, methods, classes and modules is shown in this example:

import matplotlib
matplotlib.pyplot.axhline
matplotlib.pyplot.axvline
matplotlib.pyplot.axline
matplotlib.axes.Axes.axhline
matplotlib.axes.Axes.axvline
matplotlib.axes.Axes.axline

Out:

<function Axes.axline at 0x7f73be8ef820>