Adding a Horizontal Average Line to a Plot in Matplotlib
Before we begin with adding an average line to a plot in Matplotlib, let us first prepare a sample Matplotlib scatterplot. Here is the code for the same:
Python3
#import pandasimport matplotlib.pyplot as pltimport pandas as pd# prepare the dataframedf = pd.DataFrame({'x': [2, 4, 5, 7, 9, 10, 11, 13, 14, 16], 'y': [0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9]})# import pyplot from matplotlib# create scatter plotplt.scatter(df.x, df.y)# see the plotplt.show() |
Output:
output
Now, we can use the following piece of code to add a horizontal average line to the plot:
Python3
#import numpy import numpy as np#add horizontal average line plt.axhline(y=np.nanmean(df.y)) |
Here is the complete code to add an average line to a plot in matplotlib:
Python3
#import pandas import pandas as pd#import numpy import numpy as np#prepare the dataframedf = pd.DataFrame({'x' : [2, 4, 5, 7, 9, 10, 11, 13, 14, 16], 'y' : [0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9]})#import pyplot from matplotlib import matplotlib.pyplot as plt #create scatter plot plt.scatter(df.x, df.y)#add horizontal average line plt.axhline(y=np.nanmean(df.y))#see the plot plt.show() |
Output:
output
You can see that the line is drawn at the mean of all values of Y axis, that is 1. You can use this Python code to check the mean of the values on the Y axis:
Python3
print(np.nanmean(df.y)) |
Output:
1.0
Let us now understand how the code for adding the horizontal line works. The code has two main functions, axhline and nanmean. Let us look at them in detail.
The axhline function
The axhline function stands for axis horizontal line and as per its name, it draws a horizontal line on the plot across the axes.
Python3
matplotlib.pyplot.axhline(y=0, xmin=0, xmax=1, **kwargs) |
In the code, we have passed only one argument that gives the y position of the line. You can further use other parameters to change the line color, style, and width of the line. Here is the code and corresponding output for the same:
Python3
#import pandas import pandas as pd#import numpy import numpy as np#prepare the dataframedf = pd.DataFrame({'x' : [2, 4, 5, 7, 9, 10, 11, 13, 14, 16], 'y' : [0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9]})#import pyplot from matplotlib import matplotlib.pyplot as plt #create scatter plot plt.scatter(df.x, df.y)#add horizontal average line plt.axhline( y=np.nanmean(df.y), color = 'g', linestyle = '--', linewidth = 3)#see the plot plt.show() |
Output
output
You can see that this time, the horizontal average line is green in colour and dotted in style. It is also a lot thicker than usual. You can learn more about the axhline function here.
The nanmean function
The nanmean function is used to calculate the average for a given data. The term ‘nan’ in the nanmean function signifies that this function takes care of NAN or Not a number values as well.
Python3
numpy.nanmean(a, axis=None, dtype=None, out=None, keepdims=<no value>, *, where=<no value>) |
In our code, we have only passed one parameter, i.e, ‘df.y’ to pass the y coordinates of the data.
If your data has no NAN values, then you can simply use the mean() function as follows:
Python3
#add horizontal average line plt.axhline(y=np.mean(df.y)) |
How to Add an Average Line to Plot in Matplotlib
In this article, we will learn how we can add an average line to a plot in matplotlib. We will discuss the steps to add a horizontal average line using the axhline function, as well as the steps to add a vertical average line using the axvline function in Matplotlib. Throughout the article, we will use examples and code so that you can easily implement the learnings.
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