title: stacking columns in data.frame into one column in R
body: <p>I'm having trouble stacking columns in a data.frame into one column. 
  Now my data looks something like this:</p>
  
  <pre><code>id   time    black   white   red 
  a     1       b1      w1     r1
  a     2       b2      w2     r2
  a     3       b3      w3     r3
  b     1       b4      w4     r4
  b     2       b5      w5     r5
  b     3       b6      w6     r6
  </code></pre>
  
  <p>I'm trying to transform the data.frame so that it looks like this:</p>
  
  <pre><code>id   time  colour 
  a     1     b1
  a     2     b2
  a     3     b3
  b     1     b4
  b     2     b5
  b     3     b6
  a     1     w1
  a     2     w2
  a     3     w3
  b     1     w4
  b     2     w5
  b     3     w6
  a     1     r1
  a     2     r2
  .     .     .
  .     .     .
  .     .     .
  </code></pre>
  
  <p>I'm guessing that this problem requires using the reshape package, but I'm not exactly sure how to use it to stack multiple columns under one column. Can anyone provide help on this?</p>