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Python | Marking Regions across Horizontal Axis in Plot

In this tutorial, we are going to mark regions across the horizontal axis in a plot for better distinguishing it from the rest of the plot.
Submitted by Anuj Singh, on August 01, 2020

It is a small plotting/data visualization technique used often and has a good application in areas of Physics Plotting. The following example shows an example of region marking by using inbuilt functions in Matplotlib.

Python code for marking regions across horizontal axis in plot

import numpy as np
import matplotlib.pyplot as plt

x = np.arange(0, 4 * np.pi - 3, 0.01)
y = np.sin(x)

for i in range(len(y)):
    if y[i] <= 0:
        y[i] = 0

plt.figure(figsize=(6,3))
plt.plot(x, y, color='black')
threshold = 0.75
plt.axhline(threshold, color='purple', lw=2, alpha=0.7)
plt.fill_between(x, 0, 1, where=y > threshold, color='purple', alpha=0.5)
plt.xlabel('Time(s)')
plt.ylabel('Magnitude')
plt.axhline(y=0, color='grey')
plt.grid()
plt.title('AC Wave')

plt.figure(figsize=(6,7))
plt.subplot(211)
plt.plot(x, y, color='black')
plt.xlabel('Time(s)')
plt.ylabel('Magnitude')
plt.axhline(y=0, color='grey')
plt.grid()
plt.title('AC Wave')

plt.subplot(212)
plt.plot(x, y, color='black')
threshold = 0.75
plt.axhline(threshold, color='purple', lw=2, alpha=0.7)
plt.fill_between(x, 0, 1, where=y > threshold, color='purple', alpha=0.5)
plt.xlabel('Time(s)')
plt.ylabel('Magnitude')
plt.axhline(y=0, color='grey')
plt.grid()
plt.suptitle('Comparing Plots')

Output:

Output is as figure