How to Draw Circles and Arcs with Python Library Matplotlib

Table of contents

Math Drawing and Algorithm Simulation

In article “HackerRank Robot Walks and Project Euler 208 Solutions” I drew the robot trajectory (circles and arcs) with math10. The tool is pretty nice, however not as flexible and customizable as a specialized drawing library.

Python’s matplotlib and StdDraw from Princeton’s Algorithm 4th Edition are great tools for scientific drawing and algorithm simulations.

Drawing Robot Trajectory with Matplotlib

How to draw arcs with matplotlib? It’s actually pretty straight forward.

I drew the robot trajectory arcs with python code below. We can easily use loops to simplify the drawing tasks.

import numpy
import matplotlib.pyplot as plt
from matplotlib.patches import Arc
from math import pi, cos, sin

fg, ax = plt.subplots(1, 1, figsize=(8, 8))

angle = 2 * pi / 5
mirror = (2 * cos(angle), 2 * sin(angle))
arcs = [Arc((0, 0), 2, 2, angle=0, theta1=0, theta2=72, edgecolor='blue', linewidth=2),
        Arc((0, 0), 0.4, 0.4, angle=0, theta1=0, theta2=72, edgecolor='orange', linewidth=2),
        Arc((mirror[0], mirror[1]), 2, 2, angle=0, theta1=180, theta2=252, edgecolor='blue', linewidth=2)]

# Add the arc to the axes
for a in arcs:
    ax.add_patch(a)

# Set the aspect ratio of the plot to 'equal' to ensure a circular arc
ax.set_aspect('equal')
ax.set_xticks(numpy.arange(-3, 3, 0.5))
ax.set_yticks(numpy.arange(-1, 2, 0.5))

# Set the limits of the plot
ax.set_xlim(-1, 2)
ax.set_ylim(-1, 2.2)

plt.text(0.2, 0.2, '72°', size='14')
plt.text(-0.25, -0.2, '(0,0)', size='14')

plt.plot([0, 1], [0, 0], 'b-')
plt.plot([0, 2 * cos(angle)], [0, 2 * sin(angle)], 'b-')
plt.grid()
# Show the plot
fg.canvas.draw()

And the result image is below.