Overlaying data on images in Matplotlib

Introduction

Overlaying data on images helps you see extra information on top of pictures. It makes it easier to understand patterns or highlight important parts.

You want to show points or lines on a photo, like marking locations on a map.

You need to compare data with a background image, like showing weather data on a satellite image.

You want to highlight areas of interest on a medical scan.

You want to add labels or markers on a chart that uses an image as background.

Syntax

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.image as mpimg

img = mpimg.imread('image.png')
plt.imshow(img)
plt.plot(x_values, y_values, 'ro')  # example: red dots
plt.show()

plt.imshow() shows the image.

plt.plot() adds data points or lines on top.

Examples

This example shows blue dots at two points on the image.

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.image as mpimg

img = mpimg.imread('image.png')
plt.imshow(img)
plt.plot([50, 100], [30, 80], 'bo')  # blue dots
plt.show()

This example draws a green line between two points on the image.

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.image as mpimg

img = mpimg.imread('image.png')
plt.imshow(img)
plt.plot([20, 80], [40, 90], 'g-')  # green line
plt.show()

This example uses scatter to add yellow points on the image.

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.image as mpimg

img = mpimg.imread('image.png')
plt.imshow(img)
plt.scatter([60, 120], [50, 100], color='yellow')  # yellow scatter points
plt.show()

Sample Program

This program creates a gray image and overlays red points and a blue line on it. It shows how to combine image display and data plotting.

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

# Create a simple image (a gray square)
img = np.full((100, 100), 0.7)  # gray background

plt.imshow(img, cmap='gray')

# Overlay red circle points
x_points = [20, 50, 80]
y_points = [30, 60, 90]
plt.plot(x_points, y_points, 'ro', label='Red points')

# Overlay a blue line
plt.plot([10, 90], [10, 90], 'b-', label='Blue line')

plt.legend()
plt.title('Overlaying data on a gray image')
plt.show()

Important Notes

Make sure the data coordinates match the image size to place points correctly.

You can use different colors and markers to distinguish data on the image.

Use plt.imshow() first, then add data with plt.plot() or plt.scatter().

Summary

Overlaying data on images helps combine pictures with extra information.

Use plt.imshow() to show the image and plt.plot() or plt.scatter() to add data.

Check coordinates so data appears in the right place on the image.