title: Gnuplot: set the start point for X-axis labels for timestamped data
body: <p>In Gnuplot, I'm plotting timestamped values. The data looks like this:</p>
  
  <pre><code>2019-06-01 18:23:56, 508.56
  2019-06-07 18:23:56, 508.56
  2019-06-08 13:00:00, 492.95
  ...
  </code></pre>
  
  <p>I don't want the xtics labels to be too dense, so I print them only every 2 weeks:</p>
  
  <pre><code>set xdata time
  set timefmt "%Y-%m-%d %H:%M:%S"
  set xtics format "%m/%d"
  set xtics 3600*24*15
  unset mxtics
  </code></pre>
  
  <p>It works, the labels are in 2-week intervals, but they're not pleasingly aligned to the beginning of a month. The xtics labels I see are: 06/13, 06/28, 07/13 etc. I'd like to start the labeling from the first day of a month, then the 15th of a month etc. My actual data starts on 2019-06-07. I added a fake first entry for 2019-06-01 (see above), but I get the same labeling sequence (06/13, 06/28 etc). Is there a way to set where the first xtics point should be (06/01 or 06/15) without having to manually specify "xticslabels" in the data file?</p>