Plotting a horizontal line on multiple subplots in Python using pyplot

To plot a horizontal line on multiple subplots in Python, we can use subplots() to create multiple axes and the axhline() method to draw horizontal lines across each subplot.

Steps

• Create a figure and a set of subplots using plt.subplots()

• Use the axhline() method on each axis to draw horizontal lines

• Customize line properties like color, width, and position

• Display the figure using plt.show()

Example

Here's how to create three subplots with horizontal lines of different colors and widths −

from matplotlib import pyplot as plt

# Create figure with 3 subplots
fig, (ax1, ax2, ax3) = plt.subplots(3, figsize=(7, 6))

# Draw horizontal lines on each subplot
ax1.axhline(y=0.5, color="black", linewidth=2, label="Black Line")
ax2.axhline(y=0.5, color="red", linewidth=3, label="Red Line") 
ax3.axhline(y=0.5, color="blue", linewidth=4, label="Blue Line")

# Add some sample data for context
ax1.plot([0, 1, 2, 3], [0.2, 0.8, 0.3, 0.7], 'o-', alpha=0.7)
ax2.plot([0, 1, 2, 3], [0.1, 0.9, 0.4, 0.6], 's-', alpha=0.7)
ax3.plot([0, 1, 2, 3], [0.3, 0.7, 0.2, 0.8], '^-', alpha=0.7)

# Set titles for each subplot
ax1.set_title("Subplot 1")
ax2.set_title("Subplot 2") 
ax3.set_title("Subplot 3")

# Add legends
ax1.legend()
ax2.legend()
ax3.legend()

plt.tight_layout()
plt.show()

Key Parameters of axhline()

• y − The y-coordinate where the line will be drawn

• xmin, xmax − Start and end points along x-axis (0 to 1 by default)

• color − Line color (can use names like 'red' or hex codes)

• linewidth − Thickness of the line

• linestyle − Style like '--', ':', '-.' etc.

Advanced Example with Custom Positioning

You can also control the horizontal extent of lines using xmin and xmax parameters −

from matplotlib import pyplot as plt
import numpy as np

# Create 2x2 subplots
fig, ((ax1, ax2), (ax3, ax4)) = plt.subplots(2, 2, figsize=(10, 6))

# Different horizontal line configurations
ax1.axhline(y=0, color='green', linewidth=2, linestyle='--')
ax1.set_title("Full Width Dashed Line")

ax2.axhline(y=0.5, xmin=0.2, xmax=0.8, color='orange', linewidth=3)
ax2.set_title("Partial Width Line")

ax3.axhline(y=-1, color='purple', linewidth=1)
ax3.axhline(y=1, color='purple', linewidth=1)
ax3.set_title("Multiple Horizontal Lines")

ax4.axhline(y=0, color='red', linewidth=2, alpha=0.5)
ax4.set_title("Semi-transparent Line")

# Add some data to make lines more visible
for ax in [ax1, ax2, ax3, ax4]:
    x = np.linspace(-2, 2, 50)
    y = np.sin(x)
    ax.plot(x, y, 'b-', alpha=0.7)
    ax.grid(True, alpha=0.3)

plt.tight_layout()
plt.show()

Conclusion

The axhline() method provides an easy way to add horizontal reference lines across multiple subplots. Use parameters like color, linewidth, and xmin/xmax to customize the appearance and positioning of your horizontal lines.