title: Add pandas Series to a DataFrame, preserving index
body: <p>I have been having some problems adding the contents of a pandas Series to a pandas DataFrame. I start with an empty DataFrame, initialised with several columns (corresponding to consecutive dates).</p>
  
  <p>I would like to then sequentially fill the DataFrame using different pandas Series, each one corresponding to a different date. However, each Series has a (potentially) different index.</p>
  
  <p>I would like the resulting DataFrame to have an index that is essentially the union of each of the Series indices.</p>
  
  <p>I have been doing this so far:</p>
  
  <pre><code>for date in dates:
       df[date] = series_for_date
  </code></pre>
  
  <p>However, my df index corresponds to that of the first Series and so any data in successive Series that correspond to an index 'key' not in the first Series are lost.</p>
  
  <p>Any help would be much appreciated!</p>
  
  <p>Ben</p>