Matplotlib Axes.axhline() Function in Python

The matplotlib.axes.Axes.axhline() function allows you to add horizontal lines across your plot’s axes. This function is essential for creating reference lines, thresholds, or marking key values in your visualizations.

Axes Class in Matplotlib

The Axes class is one of the core components in Matplotlib. It holds most of the figure elements such as Axis, Tick, Line2D, Text, and Polygon, and defines the coordinate system. Each Axes instance supports callbacks, enabling flexible plotting and interactions.

Syntax: Axes.axhline(self, y=0, xmin=0, xmax=1)

Parameters:

• y: The y-coordinate of the horizontal line (in data coordinates). Defaults to 0.
• xmin: The starting point of the horizontal line on the x-axis. Ranges from 0 (left) to 1 (right), with a default of 0.
• xmax: The ending point of the horizontal line on the x-axis. Similar to xmin, it ranges from 0 to 1, with a default of 1.

Returns: A list of Line2D objects representing the horizontal lines.

How to Use axhline() in Matplotlib?

Example 1: Basic Usage

import matplotlib.pyplot as plt

plt.plot([1, 2, 3, 4], [10, 20, 25, 30])
plt.axhline(y=20, color='r', linestyle='--', linewidth=2)

plt.show()

Output:

Example 2: Plot with Multiple Elements

# Implementation of matplotlib function
import matplotlib.pyplot as plt
import matplotlib.tri as mtri
import numpy as np

fig, ax = plt.subplots()
x = np.arange(0, 8 * np.pi, 0.01)
y = np.sin(x)
ax.plot(x, y, color ='black')

threshold = 0.35
ax.axhline(threshold, color ='green',lw = 3, alpha = 0.7)

ax.fill_between(x, 0, 1, where = y > threshold,
                color ='green', alpha = 0.8, 
                transform = ax.get_xaxis_transform())

ax.set_title('matplotlib.axes.Axes.axhline() Example')
plt.show()

Output:

A sine wave plot with a green threshold line at y=0.35.

axhline() function in Matplotlib adds horizontal lines to plots. By adjusting its parameters, you can easily highlight key values, add reference lines, or customize your plot’s visual style.