title: Python networkx graph: Do not draw old graph together with new graph
body: <p>The below is my code:</p>
  
  <pre><code>import networkx as nx
  for i in range(2):
      G = nx.DiGraph()
      if i==0:
          G.add_edge("A", "B")
      elif i==1:
          G.add_edge("A", "C")
      import matplotlib.pyplot as plt
      nx.draw(G)
      plt.savefig(str(i)+".png")
      G.clear()
  </code></pre>
  
  <p>It should draw line AB in file 0.png and draw line AC in file 1.png. But, after I ran it. In 0.png, there is one line AB, but in 1.png, there are two lines: AB and AC. It seems that the memory for 0.png is not cleaned, although I have had "G.clear()".</p>
  
  <p>Does anybody know how to fix it?</p>