title: Pandas find max value from series of mixed data
body: <p>Am using Pandas df and in dataframe i was able to extract to series named <strong>'xy'</strong> that look like this: </p>
  
  <pre><code>    INVOICE
  2014-08-14 00:00:00
  4557
  Printing
  nan 
  Item    AMOUNT
  nan
  1   9.6 
  0
  0
  0
  9.6
  2
  11.6
  nan 
  nan
  nan
  </code></pre>
  
  <p>what i need to find is maximum value which is usually located towards the end of 'xy' pandas series i tried to convert it to string i get into problems as some of the series are string not int or float i need a good way as i am writing this script for several different files  </p>