How to Add Vertical Line at Specific Date in Matplotlib

You can use the axvline() function to along with the datetime() function to add a vertical line at a specific date in Matplotlib:

import datetime
import matplotlib.pyplot as plt

plt.axvline(datetime.datetime(2023, 1, 5))

This particular example adds a vertical line at 1/5/2023 on the x-axis of a plot in Matplotlib.

The following example shows how to use this syntax in practice.

Example: Add Vertical Line at Specific Date in Matplotlib

Suppose we have the following pandas DataFrame that contains information about the total sales made on eight consecutive days at some company:

import datetime
import numpy as np
import pandas as pd

#create DataFrame
df = pd.DataFrame({'date': np.array([datetime.datetime(2020, 1, i+1)
                                     for i in range(8)]),
                   'sales': [3, 4, 4, 7, 8, 9, 14, 17]})

#view DataFrame
print(df)

        date  sales
0 2023-01-01      3
1 2023-01-02      4
2 2023-01-03      4
3 2023-01-04      7
4 2023-01-05      8
5 2023-01-06      9
6 2023-01-07     14
7 2023-01-08     17

We can use the following code to create a plot of sales by day and add a vertical line at the date 1/5/2023 on the x-axis:

import matplotlib.pyplot as plt

#plot sales by date
plt.plot_date(df.date, df.sales)

#rotate x-axis ticks 45 degrees and right-aline
plt.xticks(rotation=45, ha='right')

#add vertical line at 1/5/2023
plt.axvline(datetime.datetime(2023, 1, 5))

Matplotlib add vertical line at specific date

Notice that a vertical line has been added to the plot at the date 1/5/2023 on the x-axis.

Also note that you can use the color, linewidth, and linestyle arguments to customize the appearance of the line:

import matplotlib.pyplot as plt

#plot sales by date
plt.plot_date(df.date, df.sales)

#rotate x-axis ticks 45 degrees and right-aline
plt.xticks(rotation=45, ha='right')

#add customized vertical line at 1/5/2023
plt.axvline(datetime.datetime(2023, 1, 5), color='red', linewidth=3, linestyle='--')