title: Group by pandas
body: <p>I have data </p>
  
  <pre><code>mm_id                           11.02.2016  12.02.2016  13.02.2016  14.02.2016  15.02.2016  16.02.2016  17.02.2016
  b2b5b93270cb217310b0719946b0afee    0            0          0           242         0           0           0
  71f464119a2d43834c3be0af362487bc    0            23         0           0           0           0           0   
  afad5e5f9c3ba9df885eba1557ab7c73    1            0          0           22          0           0           0
  </code></pre>
  
  <p>And I need to group it with data. But I don't know how I can transfer to another format.
  Desire output (I need to get list of unique <code>ID</code> to every <code>data</code>):</p>
  
  <pre><code>day
  2016-03-01    [00051f002f5a0c179d7ce191ca2c6401, 00102b98bd9,  ...]
  2016-03-02    [00102b98bd9e71da3cf23fd1f599408d, 0012ea90a6d,  ...]
  2016-03-03    [00051f002f5a0c179d7ce191ca2c6401, 00102b98bd9,  ...]
  </code></pre>