title: python matplotlib Agg vs. interactive plotting and tight_layout
body: <p>If I use the <code>Agg</code> backend, I'm unable to keep image windows open with <code>show()</code> (regardless of <code>block=True</code> or not)---they just close virtually immediately.  If I don't use <code>Agg</code>, then I get the warning:</p>
  
  <p><code>/Library/Python/2.7/site-packages/matplotlib-1.2.0-py2.7-macosx-10.8-intel.egg/matplotlib/tight_layout.py:225: UserWarning: tight_layout : falling back to Agg renderer warnings.warn("tight_layout : falling back to Agg renderer")</code></p>
  
  <p>Sample code:</p>
  
  <pre><code>import matplotlib as mpl
  mpl.use('Agg')      # With this line = figure disappears; without this line = warning
  import matplotlib.pyplot as plt
  import matplotlib.mlab as mlab
  import numpy as np
  
  fig = plt.figure()
  ax = fig.add_subplot(111)
  mu, sigma = 0, 0.5
  x = np.linspace(-3, 3, 100)
  plt.plot(x, mlab.normpdf(x, mu, sigma))
  fig.tight_layout()
  plt.show()
  </code></pre>
  
  <p>Is there a different backend or methodology I should be using?</p>