How to set labels in matplotlib.hlines?

The matplotlib.hlines() function draws horizontal lines across the plot. To add labels to these lines, you can use the plt.text() method or create a legend by combining multiple hlines() calls with label parameters.

Method 1: Using plt.text() for Direct Labels

This approach places text labels directly next to the horizontal lines ?

import matplotlib.pyplot as plt

# Set the figure size
plt.rcParams["figure.figsize"] = [7.00, 3.50]
plt.rcParams["figure.autolayout"] = True

# Add horizontal line with direct text label
plt.hlines(y=1, xmin=1, xmax=4, lw=7, color='orange')
plt.text(4, 1, 'y=1', ha='left', va='center', fontsize=12)

# Add another horizontal line with direct text label
plt.hlines(y=2, xmin=2, xmax=5, lw=7, color='red')
plt.text(2, 2, 'y=2', ha='right', va='center', fontsize=12)

plt.xlim(0, 6)
plt.ylim(0, 3)
plt.show()

# Displays a plot with two horizontal lines and their labels positioned directly on the plot

Method 2: Using Labels with Legend

For a cleaner approach, use the label parameter with plt.legend() ?

import matplotlib.pyplot as plt

plt.rcParams["figure.figsize"] = [7.00, 3.50]
plt.rcParams["figure.autolayout"] = True

# Add horizontal lines with labels for legend
plt.hlines(y=1, xmin=1, xmax=4, lw=7, color='orange', label='Line at y=1')
plt.hlines(y=2, xmin=2, xmax=5, lw=7, color='red', label='Line at y=2')

# Display legend
plt.legend()
plt.xlim(0, 6)
plt.ylim(0, 3)
plt.show()

# Displays a plot with two horizontal lines and a legend box showing the labels

Method 3: Combining Text Labels with Formatting

You can enhance labels with custom formatting and positioning ?

import matplotlib.pyplot as plt

plt.rcParams["figure.figsize"] = [8.00, 4.00]
plt.rcParams["figure.autolayout"] = True

# Add horizontal lines
plt.hlines(y=1.5, xmin=0.5, xmax=3.5, lw=5, color='blue', alpha=0.7)
plt.hlines(y=2.5, xmin=1.5, xmax=4.5, lw=5, color='green', alpha=0.7)

# Add formatted text labels
plt.text(3.7, 1.5, 'Baseline (y=1.5)', ha='left', va='center', 
         fontsize=10, bbox=dict(boxstyle='round', facecolor='lightblue'))
plt.text(1.3, 2.5, 'Target (y=2.5)', ha='right', va='center',
         fontsize=10, bbox=dict(boxstyle='round', facecolor='lightgreen'))

plt.xlim(0, 5)
plt.ylim(1, 3)
plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.title('Labeled Horizontal Lines')
plt.show()

# Displays a plot with formatted text labels in colored boxes next to horizontal lines

Comparison

Conclusion

Use plt.text() for direct labeling with precise control over positioning. For multiple horizontal lines, combine label parameters with plt.legend() for a cleaner appearance.