title: prevent sorting of column names in pandas - python
body: <pre><code>df = pd.DataFrame(data={'i1':['x','x','x','x','y','y','y','y'], 'i2':['f','a','w','h','f','a','w','h'], 'c1':np.random.randn(8),'c2':np.random.randn(8)})
  df.set_index(['i1','i2'],inplace=True)
  </code></pre>
  
  <p><code>df.unstack('i2')</code> sorts the column names in <code>i2</code> alphabetically. How can I prevent this sorting and keep the original sequence <code>(f a w h)</code>?</p>