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
    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(x, ymin, ymax) represents a vertical line, with parameters (x-value, starting y-coordinate, ending y-coordinate).

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
    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.axhline(0, color="red")#Horizontal line
    plt.axvline(0, color="red")#Vertical line

matplotlib.pyplot.axhline(y) draws a horizontal line across the current axes.

matplotlib.pyplot.axvline(x) draws a vertical line across the current axes.

Summary

hlines and axhline can both draw horizontal lines, but they solve slightly different problems. hlines is better when you want a line segment with explicit start and end positions, while axhline is better when you want a full-width reference line.

Similarly, vlines is useful for a bounded vertical segment, while axvline is useful for a full-height vertical reference line.

Please choose different matplotlib.pyplot functions as needed.