When plotting, you often encounter situations where you need to add dividing lines. So, which functions in Python can conveniently draw horizontal or vertical lines? Below are two functions that can easily add horizontal or vertical lines.

1. Using matplotlib.pyplot’s hlines and vlines

    ######Import required modules
    import numpy as np
    import matplotlib.pyplot as plt
    ######This is the butterfly curve part
    df = sns.load_dataset('iris')
    t = np.arange(0.0, 12*np.pi, 0.01)
    x = np.sin(t)*(np.e**np.cos(t) - 2*np.cos(4*t)-np.sin(t/12)**5)
    y = np.cos(t)*(np.e**np.cos(t) - 2*np.cos(4*t)-np.sin(t/12)**5)
    plt.figure(figsize=(8,6))

    plt.plot(x,y,color='blue',linewidth = '3')
    #######This is the dividing line part
    plt.hlines(0, -3, 3,color="red")#Horizontal line
    plt.vlines(0, -2, 3,color="red")#Vertical line

matplotlib.pyplot.hlines(y, xmin, xmax) represents a horizontal line, with parameters (y-value, starting x-coordinate of the horizontal line, ending x-coordinate of the horizontal line).

matplotlib.pyplot.vlines(y, xmin, xmax) represents a vertical line, with parameters (x-value, starting y-coordinate of the vertical line, ending y-coordinate of the vertical line).

2. Using matplotlib.pyplot’s axhline and axvline

    ######Import modules
    import numpy as np
    import matplotlib.pyplot as plt
    ######This is the butterfly curve part
    df = sns.load_dataset('iris')
    t = np.arange(0.0, 12*np.pi, 0.01)
    x = np.sin(t)*(np.e**np.cos(t) - 2*np.cos(4*t)-np.sin(t/12)**5)
    y = np.cos(t)*(np.e**np.cos(t) - 2*np.cos(4*t)-np.sin(t/12)**5)
    plt.figure(figsize=(8,6))

    plt.plot(x,y,color='blue',linewidth = '3')
    #######This is the dividing line part
    plt.axhlines(0, -3, 3,color="red")#Horizontal line
    plt.axvlines(0, -2, 3,color="red")#Vertical line

matplotlib.pyplot.axhlines(y, xmin, xmax) represents a horizontal line, with parameters (y-value, starting x-coordinate of the horizontal line, ending x-coordinate of the horizontal line).

matplotlib.pyplot.axvlines(y, xmin, xmax) represents a vertical line, with parameters (x-value, starting y-coordinate of the vertical line, ending y-coordinate of the vertical line).

Summary

matplotlib.pyplot.hlines and matplotlib.pyplot.axhlines can both draw horizontal lines, but as we can see from the figure, there are differences. The hline line does not touch the axis, while axhline does.

Similarly, vlines and axvlines are both vertical lines; one does not touch the axis, and the other does.

Please choose different matplotlib.pyplot functions as needed.