title: Calculate number values from the same column for rows with the same ID and save calculation result as a new column only for the row with latest date
body: <p>I'm new to Python and trying to calculate the % difference between two numbers from Units column for two different dates and save its result as a value in a new column (My_Calculation_Result). This value should only be present on the rows with latest date. </p>
  
  <p>((Units[where date is 2020-02-01] - Units[where date is 2020-01-25] ) / Units[where date is 2020-01-25]) * 100%</p>
  
  <p>My initial CSV file structure:</p>
  
  <pre><code>Date,       ID,  Name,   Units, 
  2020-02-01, 123, Guitar,  200,            
  2020-02-01, 456, Drums,   150,            
  2020-02-01, 789, Piano,   340,            
  2020-01-25, 123, Guitar,  980,            
  2020-01-25, 456, Drums,    3,             
  2020-01-25, 789, Piano,   300,            
  </code></pre>
  
  <p>Desired output in CSV:
  In the output file I need to add the calculation results only to the rows with latest date.</p>
  
  <pre><code>Date,       ID,  Name,   Units,  My_Calculation_Result
  2020-02-01, 123, Guitar,  200,            -79.59%
  2020-02-01, 456, Drums,   150,             49.00%
  2020-02-01, 789, Piano,   340,             11.76%
  2020-01-25, 123, Guitar,  980,            
  2020-01-25, 456, Drums,    3,             
  2020-01-25, 789, Piano,   300, 
  </code></pre>
  
  <p>Thank you for any help with this in advance!            </p>