title: error: undefined name when I use clf() in matplotlib
body: <pre><code>import time
  import matplotlib.pyplot as plt
  
  xvalues = [1,2,3,4,5,6,7,8,9]
  yvalues = [1,3,5,9,8,7,8,5,6]
  plt.xlabel('time in hours')
  plt.ylabel('ph')
  plt.plot([xvalues],[yvalues], 'ro')
  plt.axis ([0,10,0,15])
  plt.show()
  time.sleep(1)
  clf()
  </code></pre>
  
  <p>I want to make a figure with a plot, and then delete the figure after a specific time. But when I try it I get the error: undefined name on the last line where I want to delete the figure.</p>