How do you plot a vertical line on a time series plot in Pandas?

Question

• How do you plot a vertical line (vlines) in a Pandas series plot?
• I am using Pandas to plot rolling means, etc., and would like to mark important

Answer 1

matplotlib.pyplot.vlines

• For a time series, the dates for the axis must be proper datetime objects, not strings.
  • Use pandas.to_datetime to convert columns to datetime dtype.
• Allows for single or multiple locations
• ymin & ymax are specified as a specific y-value, not as a percent of ylim
• If referencing axes with something like fig, axes = plt.subplots(), then change plt.xlines to axes.xlines

plt.plot() & sns.lineplot()

from datetime import datetime
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns  # if using seaborn

plt.style.use('seaborn')  # these plots use this style

# configure synthetic dataframe
df = pd.DataFrame(index=pd.bdate_range(datetime(2020, 6, 8), freq='1d', periods=500).tolist())
df['v'] = np.logspace(0, 1, num=len(df))

# plot
plt.plot('v', data=df, color='magenta')

y_min = df.v.min()
y_max = df.v.max()

plt.vlines(x=['2020-07-14', '2021-07-14'], ymin=y_min, ymax=y_max, colors='purple', ls='--', lw=2, label='vline_multiple')
plt.vlines(x=datetime(2021, 9, 14), ymin=4, ymax=9, colors='green', ls=':', lw=2, label='vline_single')
plt.legend(bbox_to_anchor=(1.04, 0.5), loc="center left")
plt.show()

df.plot()

df.plot(color='magenta')

ticks, _ = plt.xticks()
print(f'Date format is pandas api format: {ticks}')

y_min = df.v.min()
y_max = df.v.max()

plt.vlines(x=['2020-07-14', '2021-07-14'], ymin=y_min, ymax=y_max, colors='purple', ls='--', lw=2, label='vline_multiple')
plt.vlines(x='2020-12-25', ymin=y_min, ymax=8, colors='green', ls=':', lw=2, label='vline_single')
plt.legend(bbox_to_anchor=(1.04, 0.5), loc="center left")
plt.show()

package versions

import matplotlib as mpl

print(mpl.__version__)
print(sns.__version__)
print(pd.__version__)

[out]:
3.3.1
0.10.1
1.1.0