title: Calculate daily from cumulative sum
body: <p>Is there an easy way that doesn't involve a 'manual' loop to take a cumulative sum series to a daily count series?  kind of the opposite of cumsum()?</p>
  
  <pre><code>I have the following df
  Day  CumSum  
  Mon    1
  Tue    4
  Wed    8
  Thu    16
  Fri    25
  Sat    32
  Sun    43
  </code></pre>
  
  <p>I want to fill an additional column with daily values</p>
  
  <pre><code>Day   CumSum   Daily 
  Mon     1      1      
  Tue     4      3      
  Wed     8      4      
  Thu     16     8      
  Fri     25     9      
  Sat     32     7      
  Sun     43     11     
  </code></pre>