Matplotlib Fill Tutorial

The challenge in this tutorial is to learn how to make Matplotlib fill in areas. If you’re new to Matplotlib, you might want to try the Matplotlib introduction first. If you already know the basics, then change the code below to make your plot look like the goal plot by filling in different areas with different colours. You can experiment on your own, or read the rest of the tutorial to learn the concepts you need. The progress bar shows how close you are to the goal, and the canvas differences, below right, highlight the differences in red.

If you really mess up the script, you can click the reset button to go back to the start.

Table of Contents

The Challenge

### Canvas ###
import numpy as np
import matplotlib.pyplot as plt

n = 256
X = np.linspace(-np.pi, np.pi, n, endpoint=True)
Y = np.sin(2*X)

plt.plot(X, Y+1, color='blue', alpha=1.00)
plt.plot(X, Y-1, color='blue', alpha=1.00)

plt.xlim(-np.pi, np.pi)
plt.xticks([])
plt.ylim(-2.5, 2.5)
plt.yticks([])
plt.tight_layout()
plt.show()

### Goal ###
import numpy as np
import matplotlib.pyplot as plt

n = 256
X = np.linspace(-np.pi, np.pi, n, endpoint=True)
Y = np.sin(2*X)

plt.fill_between(X, 1, Y+1, facecolor='blue', alpha=0.25)
plt.plot(X, Y+1, color='blue', alpha=1.00)

plt.fill_between(X, -1, Y-1, (Y-1) > -1, facecolor='blue', alpha=0.25)
plt.fill_between(X, -1, Y-1, (Y-1) < -1, facecolor='red', alpha=0.25)
plt.plot(X, Y-1, color='blue', alpha=1.00)

plt.xlim(-np.pi, np.pi)
plt.xticks([])
plt.ylim(-2.5, 2.5)
plt.yticks([])
plt.tight_layout()
plt.show()

How to Fill

The main thing to learn about in this tutorial is the fill_between() function. A basic call looks like plt.fill_between(x, y1, y2). That’s roughly equivalent to plt.plot(x, y1) and plt.plot(x, y2), plus it fills in the area between the two lines.

Can you find the two lines that will match the goal output?

### Canvas ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 2+3*X
Y2 = 5

plt.fill_between(X, Y1, Y2)

plt.show()
### Goal ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 2+3*X
Y2 = 3+2*X

plt.fill_between(X, Y1, Y2)

plt.show()

What to Fill

That lets you fill in an area of the plot, but the default colours are intended for line graphs. They’re a bit intense to use for filling, so you can reduce the alpha value. The lower the alpha value, the more transparent the fill.

In this example, the polynomial curves are more complex, so we don’t want the fill to be too distracting. Adjust the colour parameters to match the goal.

### Canvas ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, color='blue', alpha=1.00)

plt.show()
### Goal ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, color='green', alpha=0.50)

plt.show()

What not to Fill

If you look closely, particularly at low alpha values, you can see that the filled area has a darker edge. If you want to control the fill separately from the edge, you can use facecolor and edgecolor instead of color.

### Canvas ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, color='blue', alpha=0.30)

plt.show()
### Goal ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, facecolor='blue', alpha=0.30)

plt.show()

Where to Fill

The final trick to learn is the where parameter. You can use it to only fill part of the region. You pass in an array of true or false values that correspond to each entry in the x, y1, and y2 arrays. A nice feature of numpy is that you can write comparison expressions with the X and Y arrays to produce arrays of true and false values.

### Canvas ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, where=X < 2)
plt.xlim(-11, 11)

plt.show()
### Goal ###
import numpy as np
import matplotlib.pyplot as plt

X = np.linspace(-10, 10)
Y1 = 5*X**2 - 250
Y2 = X**3

plt.fill_between(X, Y1, Y2, where=X < 5)
plt.xlim(-11, 11)

plt.show()

Now that you have all the skills you need, can you solve the challenge at the start of the tutorial? Here’s a summary of the skills you learned:

Command Explanation
fill_between(x, y1, y2) Fill between two plots.
color= Change the fill colour.
alpha= Adjust the transparency.
facecolor= Only the filled area, not the edge.
where= Only include part of the area.

For all the details of fill_between(), read the documentation. This tutorial was inspired by the work of Nicolas P. Rougier.