Matplotlib – Add an Average Line to the Plot

In this tutorial, we will look at how to add an average line to a matplotlib plot with the help of some examples.

What is an average line on a plot?

Generally, an average line is a (horizontal) line that represents the average value of the data points on the y-axis. Thus, to plot an average line (irrespective of the tool or library), first, find out the average y-value and then plot a horizontal line for that y-value.

Average line in matplotlib

To add an average line to a plot in matplotlib, you can use the matplotlib.pyplot.axhline() function and pass the average y-value as an argument. You can use the numpy.mean() function to get the average of the y-values. The following is the syntax –

import numpy as np
import matplotlib.pyplot as plt

# add an average line to a plot
plt.axhline(np.mean(y_values))

Let’s now look at some examples of using the above syntax –

First, we will create a scatter plot to which we will later add an average line.

import matplotlib.pyplot as plt

# x values - years
x = [2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020]
# y values - 1 USD in INR
y = [46.67, 53.44, 56.57, 62.33, 62.97, 66.46, 67.79, 70.09, 70.39, 76.38]

# plot x and y on scatter plot
plt.scatter(x, y)

# add axes labels
plt.xlabel('Year')
plt.ylabel('1USD in INR')

Output:

The above scatter plot depicts the price of 1 USD in INR from 2011 to 2020. Let’s add an average line to this plot, which will depict the average USD to INR conversion rate from 2011 to 2020.

import numpy as np

# plot x and y on scatter plot
plt.scatter(x, y)

# add axes labels
plt.xlabel('Year')
plt.ylabel('1USD in INR')

# add an average line
plt.axhline(np.mean(y))

Output:

You can see that the horizontal line is around 63.

You can customize the horizontal line with additional parameters. For example, let’s make the line dashed and give it a green color.

import numpy as np

# plot x and y on scatter plot
plt.scatter(x, y)

# add axes labels
plt.xlabel('Year')
plt.ylabel('1USD in INR')

# add an average line
plt.axhline(np.mean(y), linestyle='dashed', color='g')

Output:

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