title: create boxplot from matrix with ggplot2
body: <p>I have been looking for this subject but I couldn't find a clear answer to my problem. 
  I have a matrix like this: </p>
  
  <pre><code>    &gt; myMatrix
             [,1]      [,2]         [,3]
   [1,] 1.0000000 0.8236821 5.174437e-01
   [2,] 0.2696589 0.1854324 1.000000e+00
   [3,] 0.3266629 0.1982345 1.000000e+00
   [4,] 1.0000000 0.7766927 1.807857e-01
   [5,] 1.0000000 0.5033586 3.662552e-01
   [6,] 1.0000000 0.9419056 4.989487e-02
   [7,] 1.0000000 0.6215466 9.446507e-01
   [8,] 1.0000000 0.9483751 8.328207e-01
   [9,] 1.0000000 0.4597556 1.756653e-01
  [10,] 1.0000000 0.4842302 8.154578e-01
  [11,] 1.0000000 0.5108767 1.912999e-01
  [12,] 1.0000000 0.4315146 2.921003e-01
  [13,] 1.0000000 0.8404401 8.960460e-03
  [14,] 0.8190788 1.0000000 3.551697e-08
  [15,] 1.0000000 0.5707248 1.897027e-03
  [16,] 0.7768339 1.0000000 6.486805e-07
  [17,] 0.7258489 1.0000000 3.463566e-06
  [18,] 0.1839966 0.1848062 1.000000e+00
  [19,] 0.3061347 0.1304444 1.000000e+00
  [20,] 1.0000000 0.4425222 9.216534e-02
  </code></pre>
  
  <p>I would like to produce a boxplot using <code>ggplot2</code> which display this (3 groups which are the 3 columns):</p>
  
  <pre><code>boxplot(myMatrix)
  </code></pre>
  
  <p>I can't find the way with <code>ggplot2</code> to produce the same boxplot with my 3 groups.
  Please, can someone help me? </p>