title: How to update color of streamplot in Matplotlib?
body: <p>If have a streamplot:</p>
  
  <pre><code>c = streamplot(X, Y, U, V, color=(0,0,0,1))
  </code></pre>
  
  <p>Now, I want to hide it:</p>
  
  <pre><code>c.lines.set_alpha(0)
  c.arrows.set_alpha(0)
  draw()
  </code></pre>
  
  <p>The lines disapear, but not the arrows. Why? How can I change the alpha of the arrows? ('Im using qt4agg backend).</p>