How to rotate Matplotlib annotation to match a line?

To rotate Matplotlib annotation to match a line's angle, you need to calculate the line's slope and convert it to the appropriate rotation angle. This ensures text appears parallel to the plotted line.

Steps to Rotate Annotation

• Create a figure and add subplot using figure() and add_subplot() methods
• Initialize slope (m) and intercept (c) for the line equation
• Generate x and y data points using NumPy
• Calculate rotation angle using arctan() of the slope
• Plot the line and add rotated text annotation

Example

Here's how to rotate annotation to match a line's angle −

import numpy as np
import matplotlib.pyplot as plt

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

# Create figure and subplot
fig = plt.figure()
ax = fig.add_subplot()

# Line parameters
m = 1  # slope
c = 1  # intercept

# Generate data points
x = np.linspace(-2, 2, 10)
y = m * x + c

# Calculate rotation angle in degrees
theta = np.arctan(m)
rotation_angle = np.degrees(theta)

# Plot the line
line, = ax.plot(x, y, 'b-', linewidth=2)

# Add rotated annotation
ax.text(x=x[2], y=y[2]+0.25, s="y=mx+c", 
        rotation=rotation_angle, fontsize=15, color='green',
        ha='center', va='bottom')

plt.grid(True, alpha=0.3)
plt.show()

# This displays a plot with a diagonal line and text "y=mx+c" 
# rotated to match the line's angle

Understanding the Rotation Calculation

The key is using np.arctan(slope) to get the angle in radians, then converting to degrees −

import numpy as np

# Example with different slopes
slopes = [0, 0.5, 1, 2, -1]

for slope in slopes:
    angle_radians = np.arctan(slope)
    angle_degrees = np.degrees(angle_radians)
    print(f"Slope: {slope:4.1f} ? Angle: {angle_degrees:6.1f}°")

Slope:  0.0 ? Angle:    0.0°
Slope:  0.5 ? Angle:   26.6°
Slope:  1.0 ? Angle:   45.0°
Slope:  2.0 ? Angle:   63.4°
Slope: -1.0 ? Angle:  -45.0°

Multiple Annotations Example

You can add multiple rotated annotations along different lines −

import numpy as np
import matplotlib.pyplot as plt

fig, ax = plt.subplots(figsize=(8, 6))

# Different line equations
lines_data = [
    {'slope': 0.5, 'intercept': 1, 'color': 'blue', 'label': 'Line 1'},
    {'slope': -0.8, 'intercept': 3, 'color': 'red', 'label': 'Line 2'},
    {'slope': 1.5, 'intercept': -1, 'color': 'green', 'label': 'Line 3'}
]

x = np.linspace(-2, 4, 100)

for line_data in lines_data:
    m = line_data['slope']
    c = line_data['intercept']
    y = m * x + c
    
    # Plot line
    ax.plot(x, y, color=line_data['color'], linewidth=2)
    
    # Calculate rotation angle
    rotation_angle = np.degrees(np.arctan(m))
    
    # Add rotated annotation
    ax.text(x[50], y[50] + 0.2, line_data['label'], 
            rotation=rotation_angle, fontsize=12, 
            color=line_data['color'], ha='center', va='bottom')

ax.grid(True, alpha=0.3)
ax.set_xlim(-2, 4)
ax.set_ylim(-2, 5)
plt.show()

# Displays multiple lines with labels rotated to match each line's angle

Key Parameters

Conclusion

Use np.arctan(slope) and np.degrees() to calculate the rotation angle for annotations. This ensures text aligns perfectly with line direction, creating professional-looking plots with properly oriented labels.