'''

==============

3D quiver plot

==============

Demonstrates plotting directional arrows at points on a 3d meshgrid.

'''

# This import registers the 3D projection, but is otherwise unused.

from mpl_toolkits.mplot3d import Axes3D # noqa: F401 unused import

import matplotlib.pyplot as plt

import numpy as np

fig = plt.figure()

ax = fig.gca(projection='3d')

# Make the grid

x, y, z = np.meshgrid(np.arange(-0.8, 1, 0.2),

np.arange(-0.8, 1, 0.2),

np.arange(-0.8, 1, 0.8))

# Make the direction data for the arrows

u = np.sin(np.pi * x) * np.cos(np.pi * y) * np.cos(np.pi * z)

v = -np.cos(np.pi * x) * np.sin(np.pi * y) * np.cos(np.pi * z)

w = (np.sqrt(2.0 / 3.0) * np.cos(np.pi * x) * np.cos(np.pi * y) *

np.sin(np.pi * z))

ax.quiver(x, y, z, u, v, w, length=0.1, normalize=True)

plt.show()