title: Max agg on string series
body: <p>How can I do max values in string column like this?</p>
  <pre class="lang-py prettyprint-override"><code>        dataframe = pandas.DataFrame.from_dict(
              {
                  &quot;DEPARTMENT_ID&quot;: [&quot;A&quot;, &quot;B&quot;, &quot;A&quot;, &quot;B&quot;],
                  &quot;SLOT_BEGIN_TIME&quot;: [&quot;2014-01-01&quot;, &quot;2014-01-02&quot;, &quot;2014-02-01&quot;, &quot;2014-02-02&quot;],
              }
          )
          dataframe[&quot;MAX_TIME&quot;] = dataframe.groupby([&quot;DEPARTMENT_ID&quot;])[&quot;SLOT_BEGIN_TIME&quot;].max()
  </code></pre>
  <p>I get NaN for dataframe[&quot;MAX_TIME&quot;].
  If it is not possible to do max on non-numeric series, can I write my own compare function?</p>