title: How to benefit from `.BY` in data.table?
body: <p>This is the explanation in manual of <code>.BY</code></p>
  
  <blockquote>
    <p><code>.BY</code> is a list containing a length 1 vector for each item in <code>by</code>. This can be useful when <code>by</code> is not known in advance. The <code>by</code> variables are also available to <code>j</code> directly by name; useful for example for titles of graphs if <code>j</code> is a plot command, or to branch with <code>if()</code> depending on the value of a group variable.</p>
  </blockquote>
  
  <p>It says "useful for example for titles of graphs if <code>j</code> is a plot command, or to branch with <code>if()</code> depending on the value of a group variable." </p>
  
  <p>But still, I'm not sure when to use.how to benefit from this <code>.BY</code>?</p>
  
  <p>Could you give one example? Thanks a lot!</p>