>>> """
=======================================================
3D voxel / volumetric plot with cylindrical coordinates
=======================================================

Demonstrates using the *x*, *y*, *z* parameters of `.Axes3D.voxels`.
"""
... 
... import matplotlib.pyplot as plt
... import matplotlib.colors
... import numpy as np
... 
... 
... def midpoints(x):
...     sl = ()
...     for i in range(x.ndim):
...         x = (x[sl + np.index_exp[:-1]] + x[sl + np.index_exp[1:]]) / 2.0
...         sl += np.index_exp[:]
...     return x
... 
... # prepare some coordinates, and attach rgb values to each
... r, theta, z = np.mgrid[0:1:11j, 0:np.pi*2:25j, -0.5:0.5:11j]
... x = r*np.cos(theta)
... y = r*np.sin(theta)
... 
... rc, thetac, zc = midpoints(r), midpoints(theta), midpoints(z)
... 
... # define a wobbly torus about [0.7, *, 0]
... sphere = (rc - 0.7)**2 + (zc + 0.2*np.cos(thetac*2))**2 < 0.2**2
... 
... # combine the color components
... hsv = np.zeros(sphere.shape + (3,))
... hsv[..., 0] = thetac / (np.pi*2)
... hsv[..., 1] = rc
... hsv[..., 2] = zc + 0.5
... colors = matplotlib.colors.hsv_to_rgb(hsv)
... 
... # and plot everything
... ax = plt.figure().add_subplot(projection='3d')
... ax.voxels(x, y, z, sphere,
...           facecolors=colors,
...           edgecolors=np.clip(2*colors - 0.5, 0, 1),  # brighter
...           linewidth=0.5)
... 
... plt.show()
...