A lot of these solutions are focusing on adding a line to the plot that fits the data. Here’s a simple solution for adding an arbitrary line to the plot based on a slope and intercept.
import matplotlib.pyplot as plt
import numpy as np
def abline(slope, intercept):
"""Plot a line from slope and intercept"""
axes = plt.gca()
x_vals = np.array(axes.get_xlim())
y_vals = intercept + slope * x_vals
plt.plot(x_vals, y_vals, '--')