title: Plotting animated quivers in Python
body: <p>I am trying to animate a vector such as wind in Python. I tried to use quiver function in pylab and in combination with matplotlib.animation from matplotlib. However, the result says <code>'QuiverKey' object is not subscriptable</code>. I think that it is because I don't understand fully about these two functions or just these two functions don't match together. Below is my code, it is actually the combination between quiver and animation functions from matplotlib.</p>
  
  <pre><code>def update_line(num, data, line):
      line.set_data(data[...,:num])
      return line,
  
  X,Y = np.meshgrid(np.arange(0,2*np.pi,.2),np.arange(0,2*np.pi,.2) )  
  U = np.cos(X)
  V = np.sin(Y)
  
  fig1 = plt.figure()
  Q = quiver( X[::3, ::3], Y[::3, ::3], U[::3, ::3], V[::3, ::3],
          pivot='mid', color='r', units='inches' )
  data = quiverkey(Q, 0.5, 0.03, 1, r'$1 \frac{m}{s}$', fontproperties={'weight': 'bold'})
  plt.axis([-1, 7, -1, 7])
  title('scales with plot width, not view')
  l, = plt.plot([], [], 'r-') 
  plt.xlabel('x')
  plt.ylabel('y')
  plt.title('test')
  line_ani = animation.FuncAnimation(fig1, update_line, 25, fargs=(data, l),
  interval=50, blit=True)
  plt.show() 
  </code></pre>