Fill Area Between Lines in Matplotlib

In this tutorial, we’ll try to understand how to fill in the area between lines in a matplotlib plot with the help of examples.

We can easily fill in the area between lines in a Matplotlib plot by using the following functions available in the matplotlib.pyplot module:

1. fill_between() – to fill the area between horizontal curves
2. fill_betweenx() – to fill the area between vertical curves

Using the matplotlib.pyplot.fill_between() method

It is a method used to fill the area between horizontal curves on a matplotlib plot.

Basic Syntax:

matplotlib.pyplot.fill_between(x, y1, y2=0, where=None, interpolate=False, step=None, *, data=None, **kwargs)

Parameters:

• x-array (length N): The x coordinates of the nodes defining the curves.
• y1-array (length N) or scalar: The y coordinates of the nodes defining the first curve.
• y2-array (length N) or scalar: The y coordinates of the nodes defining the second curve.
• where-array of bool (length N), optional: Define where to exclude some horizontal regions from being filled.
• interpolate-bool, default: False: This option is only relevant if the where parameter is used and the two curves are crossing each other.

For more details, refer this.

Examples

Now, we’ll try to understand the above methods, with some worked-out examples.

Example 1 – Filling area between two lines

import matplotlib.pyplot as plt
import numpy as np

#define x and y
x = np.arange(10,20)
y = np.arange(10,20)

#create plot of values with specified ylim
plt.ylim(0,20)
plt.plot(x,y)

#fill in area between the lines
plt.fill_between(x, y,5, color='red')

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values for the plot.
3. Then, we adjust the y limits of the plot using plt.ylim (Refer this).
4. Then, we plot the x and y values.
5. Then, we filled the area between the given x and y values. As the y2 value is 5 (a scaler value), the graph is filled from y = 5 with red color.

Example 2 – Filling area under a curve

import matplotlib.pyplot as plt
import numpy as np

#define x and y
x = np.arange(0,100,1)
y = x**3

#create plot 
plt.plot(x,y)

#fill in area between the curve and line
plt.fill_between(x, y, color='green')

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values (a cubic equation y = x^3) for the plot.
3. Then, we plot the graph.
4. Then, we fill the region below the graph (using the fill_between() function) until the x axis because the y2 is by default 0.

Example 3 – Filling area above the curve

import matplotlib.pyplot as plt
import numpy as np

#define x and y 
x = np.arange(0,10)
y = x**3

#create plot 
plt.plot(x,y)

#fill in area above the curve
plt.fill_between(x, y, np.max(y), alpha=.3)

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values (a cubic equation y = x^3) for the plot.
3. Then, we plot the graph.
4. Then, we fill the region above the graph (using the fill_between() function) by setting the y2 as the maximum value of all the y values.

Example 4 – Filling area only under specified condition

import matplotlib.pyplot as plt
import numpy as np

#define x and y 
x = np.arange(-10,10)
y = x**3

#create plot 
plt.plot(x,y)

#fill in area above the curve
plt.fill_between(x, y,color='blue', alpha=.3, where = (y>0))
plt.fill_between(x, y,color='green', alpha=.3, where=(y<=0))

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values (a cubic equation y = x^3) for the plot.
3. Then, we plot the graph.
4. Then, we fill the area which is above the x axis with blue color and the area below the x axis with green using the where parameter of the fill_between() function.

Using the matplotlib.pyplot.fill_betweenx() method

It is a method used to fill the area between vertical cuves on a matplotlib plot.

Basic Syntax:

matplotlib.pyplot.fill_betweenx(y, x1, x2=0, where=None, step=None, interpolate=False, *, data=None, **kwargs)

Parameters:

• y-array (length N): The y coordinates of the nodes defining the curves.
• x1-array (length N) or scalar: The x coordinates of the nodes defining the first curve.
• x2-array (length N) or scalar: The x coordinates of the nodes defining the second curve.
• where-array of bool (length N), optional: Define where to exclude some horizontal regions from being filled.
• interpolate-bool, default: False: This option is only relevant if where is used and the two curves are crossing each other.

For more details, refer this.

Examples

Now, we’ll try to understand the above method, with some worked-out examples.

Example 1 – Filling area between vertical lines

import matplotlib.pyplot as plt
import numpy as np

#define x and y
x = np.arange(10,20)
y = np.arange(10,20)

#create plot of values
plt.plot(x,y)

#fill in area between the lines
plt.fill_betweenx(y, 12,15, color='red')

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values for the plot.
3. Then, we plot the graph.
4. Then, we fill the area between x1=12 and x2 = 15 with all the y values specified in the plot using fill_betweenx() function.

Example 2 – Filling area above the graph

import matplotlib.pyplot as plt
import numpy as np

#define x and y
y = np.arange(0,100,1)
x = y**3

#create plot
plt.plot(x,y)

#fill in area between the curve and line
plt.fill_betweenx(y, x, color='green')

Output:

In the above example, we –

1. First, import the required modules.
2. Then, we define the x and y values (a cubic equation `x = y^3`) for the plot.
3. Then, we plot the graph.
4. Then, we fill the area above the curve in the plot using fill_betweenx() function.

You might also be interested in –

• How to Draw a Rectangle in a Matplotlib Plot?
• How to remove the legend border (frame) in Matplotlib?
• How to change the legend position in Matplotlib?