How to fill rainbow color under a curve in Python Matplotlib?

Creating a rainbow gradient effect under a curve in Matplotlib involves using the fill_between() function with multiple color layers. This technique creates visually appealing plots by stacking colored regions with slightly different y-offsets.

Basic Rainbow Fill Implementation

The key is to plot multiple fill_between() layers, each with a different color and y-offset ?

import numpy as np
import matplotlib.pyplot as plt

def plot_rainbow_under_curve():
    rainbow_colors = ['violet', 'indigo', 'blue', 'green', 'yellow', 'orange', 'red']
    x = np.linspace(-5, 5, 100)
    y = x ** 2
    
    for i in range(len(rainbow_colors)):
        plt.fill_between(x, y + (len(rainbow_colors) - i - 1), 
                        color=rainbow_colors[i], alpha=0.8)
    
    plt.plot(x, y, 'black', linewidth=2, label='y = x²')
    plt.title('Rainbow Fill Under Curve')
    plt.xlabel('x')
    plt.ylabel('y')
    plt.legend()

plt.figure(figsize=(10, 6))
plot_rainbow_under_curve()
plt.show()

Enhanced Version with Smooth Gradient

For a smoother rainbow effect, we can use more colors and adjust transparency ?

import numpy as np
import matplotlib.pyplot as plt

def smooth_rainbow_fill():
    # Extended rainbow colors for smoother gradient
    colors = ['#9400D3', '#4B0082', '#0000FF', '#00FF00', '#FFFF00', '#FF7F00', '#FF0000']
    
    x = np.linspace(-3, 3, 200)
    y = np.sin(x) + 2  # Sine wave shifted up
    
    # Create gradient effect
    for i, color in enumerate(colors):
        offset = (len(colors) - i - 1) * 0.3
        plt.fill_between(x, y + offset, alpha=0.7, color=color)
    
    # Add the main curve
    plt.plot(x, y, 'black', linewidth=3, label='y = sin(x) + 2')
    plt.title('Smooth Rainbow Gradient Under Sine Wave')
    plt.xlabel('x')
    plt.ylabel('y')
    plt.grid(True, alpha=0.3)
    plt.legend()

plt.figure(figsize=(12, 8))
smooth_rainbow_fill()
plt.show()

Multiple Curves with Rainbow Fill

You can apply rainbow fills to multiple curves in the same plot ?

import numpy as np
import matplotlib.pyplot as plt

def multiple_rainbow_curves():
    rainbow_colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
    x = np.linspace(0, 2*np.pi, 100)
    
    curves = [
        (np.sin(x) + 3, 'Sine Wave'),
        (np.cos(x) + 1, 'Cosine Wave')
    ]
    
    plt.figure(figsize=(12, 8))
    
    for curve_idx, (y, label) in enumerate(curves):
        for i, color in enumerate(rainbow_colors):
            offset = (len(rainbow_colors) - i - 1) * 0.2
            plt.fill_between(x, y + offset + curve_idx * 0.5, 
                           alpha=0.6, color=color)
        
        plt.plot(x, y, 'black', linewidth=2, label=label)
    
    plt.title('Multiple Curves with Rainbow Fill')
    plt.xlabel('x (radians)')
    plt.ylabel('y')
    plt.legend()
    plt.grid(True, alpha=0.3)

multiple_rainbow_curves()
plt.show()

Key Parameters

Conclusion

Rainbow fills under curves are created by layering multiple fill_between() calls with different colors and y-offsets. Adjust the alpha values and offsets to control the gradient smoothness and visual appeal.