title: how to use gnuplot to draw colored points
body: <p>how can I  use gnuplot to plot <strong>isolated points</strong>(not connected points):
  the x axis must be the first column which is the date,
  the y axis is the second columns
  the 3rd column must be the color of the point(colors are defined into ranges (ex: from 1 to 3 red,
  from 4 to 6 green...)</p>
  
  <pre><code>1999-01-19  21  0
  2009-07-01  0   1
  2008-08-20  2   1
  2008-12-18  1   1
  2004-05-12  4   1
  2009-07-29  2   1
  2008-08-07  0   1
  2006-03-08  1   1
  2004-08-31  9   2
  2001-03-27  12  2
  2009-08-19  0   2
  2010-07-14  2   3
  2009-06-24  4   3
  2009-11-11  33  4
  2010-10-13  5   4
  2012-02-22  34  4
  2011-05-11  2   5
  </code></pre>