How to draw a filled arc in Matplotlib?

To draw a filled arc in Matplotlib, you can use the fill_between() method combined with mathematical functions to create curved shapes. This technique is useful for creating semicircles, arcs, and other curved filled regions in your plots.

Steps to Create a Filled Arc

• Set the figure size and adjust the padding between and around the subplots.

• Create a figure and a set of subplots.

• Initialize variables for radius and vertical offset.

• Create x and y data points using NumPy.

• Fill the area between x and y plots using fill_between().

• Set the axis aspect ratio to "equal" for proper circle shape.

• Display the figure using show() method.

Basic Filled Arc Example

Here's how to create a filled semicircle using the equation of a circle −

import matplotlib.pyplot as plt
import numpy as np

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

fig, ax = plt.subplots(1, 1)

# Define radius and vertical offset
radius = 2.0
y_offset = -1

# Create x values from -1 to 1
x = np.arange(-1.0, 1.05, 0.05)

# Calculate y values using circle equation: y = sqrt(r² - x²)
y = np.sqrt(radius - x ** 2) + y_offset

# Fill the area between the curve and y=0
ax.fill_between(x, y, 0, alpha=0.7, color='blue')

# Set axis properties
ax.axis([-2, 2, -2, 2])
ax.set_aspect("equal")
ax.grid(True, alpha=0.3)

plt.title("Filled Arc (Semicircle)")
plt.show()

Displays a blue filled semicircle with the flat edge at y=0

Customizing Arc Appearance

You can customize the arc's color, transparency, and add multiple arcs −

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots(1, 1, figsize=(8, 6))

# Create multiple arcs with different parameters
x = np.linspace(-2, 2, 100)

# Upper semicircle
y1 = np.sqrt(4 - x**2)
ax.fill_between(x, y1, 0, alpha=0.6, color='red', label='Upper Arc')

# Lower semicircle  
y2 = -np.sqrt(4 - x**2)
ax.fill_between(x, 0, y2, alpha=0.6, color='green', label='Lower Arc')

# Set equal aspect ratio and labels
ax.set_aspect("equal")
ax.grid(True, alpha=0.3)
ax.legend()
ax.set_title("Multiple Filled Arcs")
ax.set_xlabel("X-axis")
ax.set_ylabel("Y-axis")

plt.show()

Displays two filled semicircles - red upper arc and green lower arc forming a complete circle

Creating Partial Arcs

To create arcs that span only a portion of a circle, limit the x-range −

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots(1, 1, figsize=(8, 6))

# Create a quarter-circle arc
x = np.linspace(0, 2, 50)  # Only positive x values
y = np.sqrt(4 - x**2)     # Upper semicircle equation

# Fill the quarter arc
ax.fill_between(x, y, 0, alpha=0.7, color='purple', label='Quarter Arc')

# Add styling
ax.set_aspect("equal")
ax.grid(True, alpha=0.3)
ax.legend()
ax.set_title("Partial Filled Arc (Quarter Circle)")
ax.set_xlim(-0.5, 2.5)
ax.set_ylim(-0.5, 2.5)

plt.show()

Displays a purple filled quarter-circle in the first quadrant

Key Parameters

Conclusion

Use fill_between() with circle equations to create filled arcs in Matplotlib. Remember to set set_aspect("equal") to maintain proper circular shapes and customize colors and transparency for better visualization.