Filling the region between a curve and X-axis in Python using Matplotlib

To fill the region between a curve and X-axis in Python using Matplotlib, we use the fill_between() method. This technique is useful for highlighting areas under curves, creating visualizations for statistical data, or emphasizing specific regions in plots.

Basic Syntax

The fill_between() method fills the area between two horizontal curves:

plt.fill_between(x, y1, y2, where=None, alpha=None, color=None)

Parameters

• x ? Array of x-coordinates

• y1, y2 ? Arrays defining the curves (y2 defaults to 0)

• where ? Boolean condition to specify which areas to fill

• alpha ? Transparency level (0-1)

• color ? Fill color

Example 1: Basic Fill Between Curve and X-axis

Let's create a sine wave and fill the area between the curve and X-axis ?

import matplotlib.pyplot as plt
import numpy as np

# Set the figure size
plt.rcParams["figure.figsize"] = [8.00, 5.00]
plt.rcParams["figure.autolayout"] = True

# Create x and y data points
x = np.linspace(-2*np.pi, 2*np.pi, 100)
y = np.sin(x)

# Plot the curve
plt.plot(x, y, color='blue', linewidth=2, label='sin(x)')

# Fill the region between curve and X-axis
plt.fill_between(x, 0, y, color='lightblue', alpha=0.6)

# Add labels and grid
plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.title('Area Between Sine Curve and X-axis')
plt.grid(True, alpha=0.3)
plt.legend()

# Display the plot
plt.show()

Displays a sine wave with the area between the curve and X-axis filled in light blue.

Example 2: Conditional Filling

Fill only specific regions using the where parameter ?

import matplotlib.pyplot as plt
import numpy as np

# Create data
x = np.linspace(-3, 3, 100)
y = x**2 - 2

# Plot the curve
plt.figure(figsize=(8, 5))
plt.plot(x, y, color='red', linewidth=2, label='y = x² - 2')

# Fill only positive regions
plt.fill_between(x, 0, y, where=(y > 0), color='green', alpha=0.5, label='Positive area')

# Fill only negative regions
plt.fill_between(x, 0, y, where=(y < 0), color='red', alpha=0.5, label='Negative area')

# Add horizontal line at y=0
plt.axhline(y=0, color='black', linewidth=1)

plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.title('Conditional Area Filling')
plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Shows a parabola with positive areas filled in green and negative areas filled in red.

Example 3: Multiple Curves

Fill the area between two different curves ?

import matplotlib.pyplot as plt
import numpy as np

# Create data
x = np.linspace(0, 4, 100)
y1 = np.exp(-x)
y2 = np.exp(-2*x)

# Create the plot
plt.figure(figsize=(8, 5))
plt.plot(x, y1, color='blue', linewidth=2, label='e^(-x)')
plt.plot(x, y2, color='red', linewidth=2, label='e^(-2x)')

# Fill between the two curves
plt.fill_between(x, y1, y2, color='yellow', alpha=0.4, label='Area between curves')

plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.title('Area Between Two Exponential Curves')
plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Displays two exponential curves with the area between them filled in yellow.

Key Points

• Use alpha parameter for transparency control

• The where parameter allows conditional filling

• Default y2=0 fills between curve and X-axis

• Combine with plot() to show both curve and filled area

Conclusion

The fill_between() method in Matplotlib provides flexible options for highlighting areas under curves. Use conditional parameters and transparency settings to create informative and visually appealing plots that emphasize important data regions.