>>> """
==============
3D quiver plot
==============

Demonstrates plotting directional arrows at points on a 3D meshgrid.
"""
... 
... import matplotlib.pyplot as plt
... import numpy as np
... 
... ax = plt.figure().add_subplot(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()
...