title: gnuplot: Plot data and use max of repeated x value
body: <p>I want to plot a curve of a huge data file with two columns:</p>
  
  <p>datafile (example):</p>
  
  <pre><code>#dateYMD sum
  2014-02-01 70
  2014-02-01 85
  2014-02-01 95
  2014-02-02 116
  2014-02-02 123
  2014-02-09 130
  2014-02-09 134
  2014-02-11 145
  </code></pre>
  
  <p>if I use "plot 'data.txt' using 0:1 with lines" and set the date format etc right I'll get a plot which is nearly correct, but unfortunately it uses the "lower/upper" value if more than one line per date is given. I want to plot the maximum of the sum-column per day in my plot. I want to use the last/highest given value for each day.</p>