title: R - reformat a dataframe in order to make a ggplot histogram
body: <p>I need to reshape a dataframe in R in order to build a ggplot histogram with two bars.</p>
  <p>My dataframe is this one :</p>
  <pre><code>    year1 year2 id
  X1    2   3      3
  X2    1   3      4
  X3    4   3      5
  </code></pre>
  <p>How could I do to transform that dataframe into this one :</p>
  <pre><code>id  values  year
  3     2     year1
  3     3     year2
  4     1     year1 
  4     3     year2
  5     4     year1
  5     3     year2
  </code></pre>
  <p>I made some tries with the <em>tidyr</em> library but I didn't manage to get the result I'd like.<br />
  Any help would be very appreciated, thanks !</p>