title: How to group by 3 columns in pandas and calculate its frequency per year starting from the first entry given in date column
body: <p>I have dataframe where there are 4 columns</p>
  
  <p>I want to group by x,y,z columns and get the count of these combinations which lie in same year starting from the first entry in that grouped frame.</p>
  
  <p>I have :</p>
  
  <pre><code>x y z date
  1 2 2 08-12-2016 00:00
  1 3 2 24-03-2017 00:00
  1 2 2 22-04-2017 00:00
  1 2 2 22-04-2019 00:00
  </code></pre>
  
  <p>I want :</p>
  
  <pre><code>x y z  level count
  1 2 2  1        2
  1 3 2  1        1
  1 2 2  2        1
  </code></pre>
  
  <p>the Bin represents the Bin number in which these combinations have fallen and count represents the count of these combinations in that particular bin</p>
  
  <p>thank you</p>