title: Make gnuplot's x axis start at 0 instead of the first date
body: <p>I'm plotting some [date, value] formatted data (cpu use over time) and want the xtics to start at 0 or 1 instead of the first date. </p>
  
  <p>So for 10 hours it should look like: <code>0:01:00, 0:02:00, .., 0:10:00</code> insead of <code>9:23:00 ...</code> (since the first day in the data is the 9th at 11pm).</p>
  
  <p>My current datetime settings: </p>
  
  <pre><code>set xdata time
  set timefmt "%Y-%m-%d %H:%M:%S"
  set xtics format "%d:%H:%M" font ",25"
  </code></pre>
  
  <p><strong>Edit</strong>: If there's a way to just count hours starting from 0 on the x axis without day info that'd be awesome too. Either way. I just want to start counting from 0 to make the "time taken" very clear. </p>