# 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)

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)

plt.xlabel('Year')
plt.ylabel('1USD in INR')

plt.axhline(np.mean(y))```

Output:

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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)

plt.xlabel('Year')
plt.ylabel('1USD in INR')

plt.axhline(np.mean(y), linestyle='dashed', color='g')```

Output:

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