title: Set value for particular cell in pandas DataFrame using index
body: <p>I have created a Pandas DataFrame</p>
  <pre><code>df = DataFrame(index=['A','B','C'], columns=['x','y'])
  </code></pre>
  <p>and have got this</p>
  <pre>
      x    y
  A  NaN  NaN
  B  NaN  NaN
  C  NaN  NaN
  </pre>
  <p>Now, I would like to assign a value to particular cell, for example to row <code>C</code> and column <code>x</code>.
  I would expect to get this result:</p>
  <pre>
      x    y
  A  NaN  NaN
  B  NaN  NaN
  C  10  NaN
  </pre>
  <p>with this code:</p>
  <pre><code>df.xs('C')['x'] = 10
  </code></pre>
  <p>However, the contents of <code>df</code> has not changed. The dataframe contains yet again only <code>NaN</code>s.</p>
  <p>Any suggestions?</p>