title: Panel data in long format
body: <p>I have two data frames:</p>
  
  <p><code>d1</code>:</p>
  
  <pre><code> Id group  occu  D  Year
   12     1    1  12  2007
   13     4    2  67  2007
   14     6    3  34  2007
   15     7    1  88  2007
   16     2    2  72  2007
   17     1    1  43  2007
   18     4    1  66  2007
  </code></pre>
  
  <p>and <code>d2</code>:</p>
  
  <pre><code>    Id group  occu  D Year
     12     1    1  34  2010
     13     4    2 100  2010
     14     6    3  76  2010
     15     7    1  99  2010
     16     2    2 102  2010
     17     1    1  55  2010
     18     4    1  32  2010
  </code></pre>
  
  <p>The variables "<code>group</code>"  and "<code>occu</code>" are <code>factors</code> I want to make a <code>panel data</code> for the year 2007 and 2010 in the long form in R.</p>
  
  <p>How can I do this?</p>