How to Create Fill Between Plot in Matplotlib: Simple Guide

Use the fill_between function in matplotlib to fill the area between two curves or between a curve and a baseline. You provide the x-values and the y-values for the upper and lower boundaries, and matplotlib fills the space between them with color.

Syntax

The basic syntax of fill_between is:

• x: The x-coordinates of the points.
• y1: The y-coordinates of the first curve (lower boundary).
• y2 (optional): The y-coordinates of the second curve (upper boundary). If omitted, the area is filled between y1 and zero.
• color (optional): The fill color.
• alpha (optional): The transparency level of the fill.

python

matplotlib.pyplot.fill_between(x, y1, y2=None, color=None, alpha=None, **kwargs)

Example

This example shows how to fill the area between a sine curve and the x-axis.

python

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 2 * np.pi, 100)
y = np.sin(x)

plt.plot(x, y, label='sin(x)')
plt.fill_between(x, y, color='skyblue', alpha=0.5)
plt.title('Fill Between sin(x) and x-axis')
plt.legend()
plt.show()

Output

A plot showing the sine curve with the area between the curve and the x-axis filled in light blue.

Common Pitfalls

Common mistakes include:

• Not providing matching lengths for x, y1, and y2.
• Forgetting that if y2 is omitted, the fill is between y1 and zero.
• Using incompatible data types for coordinates.

Example of a common mistake and fix:

python

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 2 * np.pi, 100)
y1 = np.sin(x)
y2 = np.sin(x) + 1

# Wrong: y2 length mismatch (uncomment to see error)
# y2_wrong = np.sin(x[:-1]) + 1
# plt.fill_between(x, y1, y2_wrong)  # This will raise an error

# Correct usage
plt.plot(x, y1, label='sin(x)')
plt.plot(x, y2, label='sin(x)+1')
plt.fill_between(x, y1, y2, color='lightgreen', alpha=0.5)
plt.legend()
plt.title('Fill Between Two Curves')
plt.show()

Output

A plot showing two sine-based curves with the area between them filled in light green.

Quick Reference

Tips for using fill_between:

• Use alpha to control transparency.
• Use where parameter to fill conditionally.
• Combine with plot for clear boundaries.
• Colors can be named strings or hex codes.

Key Takeaways

Use matplotlib's fill_between to color the area between two curves or a curve and the x-axis.

Ensure x, y1, and y2 arrays have the same length to avoid errors.

The alpha parameter controls the transparency of the fill color.

You can fill conditionally using the where parameter for more control.

Always plot the boundary lines for better visualization alongside the fill.