Matplotlib half black and half white circle

Question

I would like to place a half black, half white circle of radius R at the origin of a matplotlib plot. I'm aware that there exists a Circle class, but I don't know how to specify that the left half of the circle should be white and the right half should be black. (An ideal solution would allow me to specify the orientation of the circle -- e.g. I should be able to rotate it so that for example the top could be white and the bottom black).

Answer 1

The easiest way is to use two Wedges. (This doesn't automatically rescale the axes, but that's easy to add, if you'd like.)

As a quick example:

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

def main():
    fig, ax = plt.subplots()
    dual_half_circle((0.5, 0.5), radius=0.3, angle=90, ax=ax)
    ax.axis('equal')
    plt.show()

def dual_half_circle(center, radius, angle=0, ax=None, colors=('w','k'),
                     **kwargs):
    """
    Add two half circles to the axes *ax* (or the current axes) with the 
    specified facecolors *colors* rotated at *angle* (in degrees).
    """
    if ax is None:
        ax = plt.gca()
    theta1, theta2 = angle, angle + 180
    w1 = Wedge(center, radius, theta1, theta2, fc=colors[0], **kwargs)
    w2 = Wedge(center, radius, theta2, theta1, fc=colors[1], **kwargs)
    for wedge in [w1, w2]:
        ax.add_artist(wedge)
    return [w1, w2]

main()

If you'd like it to always be at the origin, you can specify the transform to be ax.transAxes, and turn clipping off.

E.g.

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

def main():
    fig, ax = plt.subplots()
    dual_half_circle(radius=0.1, angle=90, ax=ax)
    ax.axis('equal')
    plt.show()

def dual_half_circle(radius, angle=0, ax=None, colors=('w','k'), **kwargs):
    """
    Add two half circles to the axes *ax* (or the current axes) at the lower
    left corner of the axes with the specified facecolors *colors* rotated at
    *angle* (in degrees).
    """
    if ax is None:
        ax = plt.gca()
    kwargs.update(transform=ax.transAxes, clip_on=False)
    center = (0, 0)
    theta1, theta2 = angle, angle + 180
    w1 = Wedge(center, radius, theta1, theta2, fc=colors[0], **kwargs)
    w2 = Wedge(center, radius, theta2, theta1, fc=colors[1], **kwargs)
    for wedge in [w1, w2]:
        ax.add_artist(wedge)
    return [w1, w2]

main()

However, this will make the "circularity" of the circle depend on aspect ratio of the outline of the axes. (You can get around that in a couple of ways, but it gets more complex. Let me know if that's what you had in mind and I can show a more elaborate example.) I also may have misunderstood what you meant "at the origin".

Answer 2

You can use the unicode half filled circle (U+25D0) if you have a font with this symbol. Oddly, this isn't in STIX (included with matplotlib), but I know it's in DejaVu Sans, so I'll use it from there.

import matplotlib.pyplot as plt
import matplotlib.font_manager
from numpy import *

path = '/full/path/to/font/DejaVuSans.ttf'
f0 = matplotlib.font_manager.FontProperties()    
f0.set_file(path)

plt.figure()
plt.xlim(-1.2,1.2)
plt.ylim(-1.2,1.2)

for angle in arange(0, 2*pi, 2*pi/10):
    x, y = cos(angle), sin(angle)
    plt.text(x, y, u'\u25D0', fontproperties=f0, rotation=angle*(180/pi), size=30)

plt.show()