title: How to group a Series by values in pandas?
body: <p>I currently have a pandas <code>Series</code> with dtype <code>Timestamp</code>, and I want to group it by date (and have many rows with different times in each group).</p>
  
  <p>The seemingly obvious way of doing this would be something similar to</p>
  
  <pre><code>grouped = s.groupby(lambda x: x.date())
  </code></pre>
  
  <p>However, pandas' <code>groupby</code> groups Series by its index. How can I make it group by value instead?</p>