title: pandas: how can I select rows in a timeseries that are "largest seen so far"?
body: <p>I have a dataframe with two columns: a timeseries and a value. I would like a function that only selects rows in the dataframe that are the "highest seen value so far", e.g., traverse the rows from oldest to youngest and only when a value is greater than any seen so far, output the row associated with that value. For the dataframe:</p>
  
  <pre><code>df=pd.DataFrame([
          ['1/10/12',10],
          ['1/11/12',11],
          ['1/12/12',13],
          ['1/14/12',12],
          ['1/9/12',3],
          ['1/8/12',4],
          ['1/18/12',12],
          ],
          columns=['date','n'])
  </code></pre>
  
  <p>what I'd want is</p>
  
  <pre><code>      date   n
  0   1/8/12   4
  1  1/10/12  10
  1  1/11/12  11
  2  1/12/12  13
  </code></pre>