How to plot a half-black and half-white circle using Matplotlib?

To create a half-black and half-white circle using Matplotlib, we use the Wedge patch class to draw two semicircular wedges with different colors. This technique is useful for creating yin-yang symbols, pie charts, or visual demonstrations.

Basic Half-Black Half-White Circle

Here's how to create a simple half-black and half-white circle using two wedges ?

import matplotlib.pyplot as plt
from matplotlib.patches import Wedge

# Set figure size
plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

# Create figure and axes
fig, ax = plt.subplots()

# Define parameters
theta1, theta2 = 0, 180  # Angles for semicircles
radius = 2
center = (0, 0)

# Create two wedges
w1 = Wedge(center, radius, theta1, theta2, fc='black', edgecolor='black')
w2 = Wedge(center, radius, theta2, theta1, fc='white', edgecolor='black')

# Add wedges to the plot
for wedge in [w1, w2]:
    ax.add_artist(wedge)

# Set equal scaling and limits
ax.axis('equal')
ax.set_xlim(-3, 3)
ax.set_ylim(-3, 3)

plt.show()

How It Works

The code creates two Wedge objects:

• w1: Black semicircle from 0° to 180° (top half)
• w2: White semicircle from 180° to 0° (bottom half)
• fc: Face color (fill color)
• edgecolor: Border color

Vertical Split Version

To create a vertically split circle instead ?

import matplotlib.pyplot as plt
from matplotlib.patches import Wedge

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

# Vertical split: left and right halves
theta1, theta2 = 90, 270  # Left half
theta3, theta4 = 270, 90  # Right half

radius = 2
center = (0, 0)

# Left half - black
w1 = Wedge(center, radius, theta1, theta2, fc='black', edgecolor='black')
# Right half - white  
w2 = Wedge(center, radius, theta3, theta4, fc='white', edgecolor='black')

ax.add_artist(w1)
ax.add_artist(w2)

ax.axis('equal')
ax.set_xlim(-3, 3)
ax.set_ylim(-3, 3)
ax.set_title('Vertically Split Circle')

plt.show()

Parameters

Conclusion

Use Wedge patches with complementary angle ranges to create half-black and half-white circles. The key is setting theta1 and theta2 to cover 180° each, ensuring the two wedges form a complete circle with different colors.