title: Replace dataframe column negative values with nan, in method chain
body: <p>I want to replace by <code>np.nan</code> all the <em>negative</em> numbers that are in column  <code>'b'</code></p>
  
  <ul>
  <li>using a method on df</li>
  <li>not in place.</li>
  </ul>
  
  <p>Here's the sample frame: </p>
  
  <pre><code>pd.DataFrame({'a': [1, 2] , 'b': [-3, 4], 'c': [5, -6]})
  </code></pre>
  
  <p>See <a href="https://stackoverflow.com/questions/27759084/how-to-replace-negative-numbers-in-pandas-data-frame-by-zero">this question</a> for in-place and non-method solutions.</p>