title: Creating a stacked bar plot with ggplot2
body: <p>I'm trying to create a stacked bar plot to display for each column x1-4 how many of the total 14 observations are -1 and how many are 1.
  I want to get them all on the same graph - 1 bar for each column.
  The numbers can be treated as factors.</p>
  
  <p>The data :</p>
  
  <pre><code>    x1   x2     x3    x4
  1   -1    1     1      1
  2   -1    1     1     -1
  3   -1    1     1      1
  4   -1    1     1      1
  5   -1   -1    -1     -1
  6    1    1     1      1
  7   -1    1     1      1
  8   -1    1    -1      1
  9   -1    1    -1      1
  10  -1    1     1      1
  11  -1   -1    -1      1
  12  -1    1    -1     -1
  13  -1   -1    -1      1
  14  -1   -1    -1     -1
  </code></pre>