Scale bars are essential features of a figure in a publication. Matplotlib doesn’t have this implemented but there is an add-on module that allows you to draw scale bars in your figures called matplotlib-scalebar.

The module is only available using pip and you can install it with

pip install matplotlib-scalebar

To insert the scale bar in your figure you have to create a scalebar patch which will be inserted in the figure:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib_scalebar.scalebar import ScaleBar

img = np.random.normal(0,1,size=(100,100))

plt.imshow(img)

pixel_size = 60/1590
scalebar = ScaleBar(pixel_size, 'mm') 
plt.gca().add_artist(scalebar)

If you use plt.subplots the same effect can be achived with

fig,ax = plt.subplots(1,2)
ax[0].imshow(img)

pixel_size = 60/1590
scalebar = ScaleBar(pixel_size, 'mm')
ax[0].add_artist(scalebar)

scalebar

Fine-tuning the scale bar

Sometimes, you don’t get what you want with the default settings for ScaleBar.

You can for example set the font size and position with this line:

scalebar = ScaleBar(pixel_size, 'mm',
                    loc='lower center', # Specify the position of the bar in the image
                    color='white',      # Set the color of text and bar
                    frameon=False,      # Add a frame 
                    size_vertical=1,    # vertical thickness of the scalebar
                    font_properties={"size": 5})