text: """
  Demo of the streamplot function with masking.
  
  This example shows how streamlines created by the streamplot function skips
  masked regions and NaN values.
  """
  import numpy as np
  import matplotlib.pyplot as plt
  
  w = 3
  Y, X = np.mgrid[-w:w:100j, -w:w:100j]
  U = -1 - X**2 + Y
  V = 1 + X - Y**2
  speed = np.sqrt(U*U + V*V)
  
  mask = np.zeros(U.shape, dtype=bool)
  mask[40:60, 40:60] = 1
  U = np.ma.array(U, mask=mask)
  U[:20, :20] = np.nan
  
  plt.streamplot(X, Y, U, V, color='r')
  
  plt.imshow(~mask, extent=(-w, w, -w, w), alpha=0.5,
             interpolation='nearest', cmap=plt.cm.gray)
  
  plt.show()
  
  