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

When working with time series data in Pandas, you often need to highlight specific dates or events by adding vertical lines to your plots. This can be achieved using matplotlib's axvline() method on the plot axes.

Creating a Time Series DataFrame

First, let's create a sample time series dataset with dates as the index ?

import pandas as pd
import matplotlib.pyplot as plt

# Create a DataFrame with date range
df = pd.DataFrame(index=pd.date_range("2019-07-01", "2019-07-31"))
df["value"] = range(1, 32)  # Sample values for each day

print(df.head())

            value
2019-07-01      1
2019-07-02      2
2019-07-03      3
2019-07-04      4
2019-07-05      5

Adding Vertical Lines to Time Series Plot

Use axvline() method to add vertical lines at specific dates ?

import pandas as pd
import matplotlib.pyplot as plt

# Create time series data
df = pd.DataFrame(index=pd.date_range("2019-07-01", "2019-07-31"))
df["value"] = range(1, 32)

# Create the plot
ax = df.plot(figsize=(10, 6))

# Add vertical lines at specific dates
ax.axvline("2019-07-15", color="green", linestyle="dashed", label="Mid-month")
ax.axvline("2019-07-25", color="red", linestyle="dashed", label="Important date")

# Customize the plot
plt.title("Time Series with Vertical Reference Lines")
plt.xlabel("Date")
plt.ylabel("Value")
plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Multiple Vertical Lines with Different Styles

You can add multiple vertical lines with various styling options ?

import pandas as pd
import matplotlib.pyplot as plt

# Create sample data
df = pd.DataFrame(index=pd.date_range("2019-07-01", "2019-07-31"))
df["sales"] = [20, 25, 30, 28, 35, 40, 38, 42, 45, 48, 50, 52, 55, 53, 58, 
               60, 62, 65, 63, 68, 70, 72, 75, 73, 78, 80, 82, 85, 83, 88, 90]

ax = df.plot(kind='line', figsize=(12, 6))

# Add different styled vertical lines
ax.axvline("2019-07-10", color="blue", linestyle="-", linewidth=2, alpha=0.7, label="Campaign Start")
ax.axvline("2019-07-20", color="orange", linestyle="--", linewidth=1.5, label="Promotion")
ax.axvline("2019-07-28", color="red", linestyle=":", linewidth=2, label="Month End")

plt.title("Sales Data with Event Markers")
plt.xlabel("Date")
plt.ylabel("Sales")
plt.legend()
plt.tight_layout()
plt.show()

Line Style Options

Conclusion

Use axvline() on your plot axes to add vertical reference lines at specific dates in time series plots. This method accepts various styling parameters like color, linestyle, and linewidth to customize the appearance of your reference lines.