CODEWITHSUNDEEP
pythonmatplotlibpolar-coordinates
Tom Johnson
Tom Johnson

Reputation: 2164

Matplotlib using Wedge() in polar plots

TL/DR: How to use Wedge() in polar coordinates?

I'm generating a 2D histogram plot in polar coordinates (r, theta). At various values of r there can be different numbers of theta values (to preserve equal area sized bins). To draw the color coded bins I'm currently using pcolormesh() calls for each radial ring. This works ok, but near the center of the plot where there may be only 3 bins (each 120 degrees "wide" in theta space), pcolormesh() draws triangles that don't "sweep" out full arc (just connecting the two outer arc points with a straight line).

I've found a workaround using ax.bar() call, one for each radial ring and passing in arrays of theta values (each bin rendering as an individual bar). But when doing 90 rings with 3 to 360 theta bins in each, it's incredibly slow (minutes).

I tried using Wedge() patches, but can't get them to render correctly in the polar projection. Here is sample code showing both approaches:

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.patches import Wedge
from matplotlib.collections import PatchCollection

# Theta coordinates in degrees

theta1=45
theta2=80

# Radius coordinates

r1 = 0.4
r2 = 0.5

# Plot using bar()

fig, ax = plt.subplots(figsize=[6,6], subplot_kw={'projection': 'polar'})
theta_mid = np.deg2rad((theta1 + theta2)/2)
theta_width = np.deg2rad(theta2 - theta1)
height = r2 - r1
ax.bar(x=theta_mid, height = height, width=theta_width, bottom=r1)
ax.set_rlim(0, 1)
plt.savefig('bar.png')

# Plot using Wedge()

fig, ax = plt.subplots(figsize=[6,6], subplot_kw={'projection': 'polar'})
patches = []
patches.append( Wedge(center=(0, 0), r = r1, theta1=theta1, theta2=theta2, width = r2-r1, color='blue'))
p = PatchCollection(patches)
ax.add_collection(p)
ax.set_rlim(0, 1)
plt.savefig('wedge.png')

The outputs of each are:

Bar

enter image description here

Wedge

enter image description here

I've tried using radians for the wedge (because polar plots usually want their angle values in radians). That didn't help.

Am I missing something in how I'm using the Wedge? If I add thousands of Wedges to my Patch collection should I have any expectation it will be faster than bar()?

Upvotes: 0

Views: 1237

Answers (1)

Tom Johnson
Tom Johnson

Reputation: 2164

Thinking this was an actual bug, I opened this issue https://github.com/matplotlib/matplotlib/issues/22717 on matplotlib where one of the maintainers nicely pointed out that I should be using Rectangle() instead of Wedge().

The solution they provided is

from matplotlib.patches import Rectangle

fig, ax = plt.subplots(figsize=[6,6], subplot_kw={'projection': 'polar'})
p = PatchCollection([Rectangle((np.deg2rad(theta1), r1), theta_width, height, color='blue')])
ax.add_collection(p)
ax.set_rlim(0, 1)
plt.savefig('wedge.png')

Upvotes: 1

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