title: gnuplot: insert custom xtic label amidst otherwise regular tic intervals
body: <p>I'm actually using orgmode to export, so my data is simply in org-table form; that shouldn't matter. It's like so:</p>
  
  <pre><code>|  Time | Video | Presentation | Paper |
  |-------+-------+--------------+-------|
  |     0 |     0 |            0 |     0 |
  |    20 |    54 |           63 |    38 |
  | 31.25 |    72 |           81 |    44 |
  |   33. |    83 |           89 |    49 |
  |   34. |    95 |          101 |    54 |
  |  36.5 |   102 |          112 |    59 |
  </code></pre>
  
  <p>This is for work, and I'm plotting file downloads over the number of hours since they were made available. An effort was made to point people toward these files at t=28hrs, and I wanted to have a custom tick there matching a vertical line so I could call attention to that and show the downloads before and after the publicity effort. Thus, my gnuplot code looks like so:</p>
  
  <pre><code>set title "File Downloads"
  
  set xlabel "Time Since Posting (hrs)"
  set xrange [0:40]
  set xtics  0,5,40
  
  set ylabel "Downloads"
  set yrange [0:125]
  set ytics 0,25,125
  
  plot data u 2:3 w l lw 2 lc 1 title 'Video', \
       data u 2:4 w l lw 2 lc 2 title 'Presentation', \
       data u 2:5 w l lw 2 lc 3 title 'Paper
  </code></pre>
  
  <p>I'm aware that one can do something like</p>
  
  <pre><code>set xtics ("0" 0, "10" 10, "Event" 30, "40" 40)
  </code></pre>
  
  <p>But I don't really want to do that for all my tics, especially since this will be an ongoing plot and I don't know how far in time I'll track things. Can I specify a regular set of intervals like the above, but then call out a custom tic on top of the axis somehow amidst those pre-defineid tics? This won't work, but for example:</p>
  
  <pre><code>set xtics 0,5,40; ("Event" 30)
  </code></pre>
  
  <p>Thanks for any suggestions.</p>