Matplotlib: How to add an Average Line to a Plot

# Table of Contents

# Matplotlib: How to add an Average Line to a Plot

Use the pyplot.axhline() method to add an average line to a Plot in Matplotlib.

The method takes the y position in data coordinates as a parameter.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

# ✅ Add an average line to a plot
plt.axhline(
    np.mean(y),
    color='r',
    linestyle='-',
    linewidth=2,
    label='Mean'
)

legend = ax.legend(loc='upper right')


plt.show()

matplotlib add average line to plot

The code for this article is available on GitHub

We called the pyplot.axhline() method with the result of calling numpy.mean().

main.py

Copied!
# ✅ Add an average line to a plot
plt.axhline(
    np.mean(y),
    color='r',
    linestyle='-',
    linewidth=2,
    label='Mean'
)

All other arguments in the call to pyplot.axhline() are optional.

The pyplot.axhline() method adds a horizontal line across the axes.

The only argument that is required is y - the y position in data coordinates of the horizontal line.

We used the numpy.mean() method to compute the arithmetic mean along the y axis.

The pyplot.axhline() method takes many optional parameters that you can specify to adjust the styling of the average line.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

plt.axhline(
    np.mean(y),
    color='g',
    linestyle='--',
    linewidth=2,
    label='Mean'
)

legend = ax.legend(loc='upper right')


plt.show()

adjust styling of average line

The code for this article is available on GitHub

You can view all of the available parameters the pyplot.axhline() method takes in the "Other Parameters" section.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

print(np.mean(y))

plt.axhline(
    np.mean(y),
    color='r',
    linestyle=':',
    linewidth=1.5,
    label='Mean',
)

legend = ax.legend(loc='upper right')


plt.show()

adjusting the optional parameters

The code for this article is available on GitHub

# Matplotlib: Adding a vertical line to denote the mean

If you need to add a vertical line to denote the mean, use the pyplot.axvline() method.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

print(np.mean(y))

plt.axvline(
    np.mean(x),
    color='red',
    linestyle='-',
    linewidth=2,
    label='Mean',
)

legend = ax.legend(loc='upper right')


plt.show()

add vertical line to denote the mean in matplotlib

The code for this article is available on GitHub

The pyplot.axvline() method adds a vertical line across the axes.

The only required argument we passed to axvline() is x - the x position in data coordinates of the vertical line.

You can adjust the way the vertical line looks by supplying some of the optional parameters the method takes.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

print(np.mean(y))

plt.axvline(
    np.mean(x),
    color='red',
    linestyle='--',
    linewidth=2,
    label='Mean',
)

legend = ax.legend(loc='upper right')


plt.show()

adjust way vertical line looks

You can view all of the parameters the axvline() method takes in the Other Parameters section.

# Matplotlib: Adding an Average Line to a Plot with a second `axes.plot()` call

You can also add an average line to a plot with a second axes.plot() call.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)

ax = plt.subplot(111)

ax.plot(x, y, label='Data')

# ✅ 1) Calculate the mean of `y`
y_mean = [np.mean(y)] * len(x)

# ✅ 2) Plot the average line
mean_line = ax.plot(
    x,
    y_mean,
    label='Mean',
    linestyle='-',
    color='red'
)

legend = ax.legend(loc='upper right')


plt.show()

add average line to plot with second axes plot call

The code for this article is available on GitHub

We first calculate the mean of y and then pass the x and y_mean variables to axes.plot().

You can adjust how the average line looks by passing different parameters to the axes.plot() method.

main.py

Copied!
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 4*np.pi, 150)
y = np.sin(x)



ax = plt.subplot(111)

ax.plot(x, y, label='Data')

# ✅ 1) Calculate the mean of `y`
y_mean = [np.mean(y)] * len(x)

# ✅ 2)️ Plot the average line
mean_line = ax.plot(
    x,
    y_mean,
    label='Mean',
    linestyle='--',
    color='green',
    linewidth=2,
)

legend = ax.legend(loc='upper right')


plt.show()

passing different parameters to axes plot method

You can view all of the available arguments the axes.plot() method takes in the Other Parameters section in the docs.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

I wrote a book in which I share everything I know about how to become a better, more efficient programmer.

You can use the search field on my Home Page to filter through all of my articles.

Matplotlib: How to add an Average Line to a Plot

# Table of Contents