title: kde density plot using python pandas
body: <p>I was following the "python for data analysis" in ipython, trying to do a kde plot using pandas(chapter 8, Histograms and Density Plots).the codes are simple:</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  ser = pd.Series(np.random.randn(1000))
  ser.plot(kind='kde')
  </code></pre>
  
  <p>the error is  </p>
  
  <pre><code>UnboundLocalError:local variable 'klass' referenced before assignment
  </code></pre>
  
  <p>I googled around and I think it is a local&amp;global variable problem and may be due to a wrong way to import the modules. Anybody can suggest any ideas?</p>