title: Python candlestick chart can not show
body: <p>I executed the code below and no errors showed in the process, but plt.show() could not work and showed nothing in the end! I'm confused and want to know why...... </p>
  
  <pre><code>from mpl_finance import candlestick_ohlc
  
  import matplotlib.dates as mdates
  
  import matplotlib.pyplot as plt
  
  import quandl
  
  df = quandl.get("EOD/AAPL", authtoken="fzTPb-TWywaPkbdAS1VF")
  
  df['Date'] = df.index.map(mdates.date2num)
  
  ohlc = df[['Date','Open','High','Low','Close']]
  
  f1, ax = plt.subplots(figsize = (10,5))
  
  candlestick_ohlc(ax, ohlc.values, width=.6, colorup='green', colordown='red')
  ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m'))
  
  plt.show()
  </code></pre>