title: Adding Horizontal lines on 2D plot with gnuplot
body: <p>I am creating a 2D plot from several data files (snapshot_XXXX.dat) using the following snippet</p>
  
  <pre><code>set terminal pngcairo
  n=1000  # Number of snapshots
  
  load 'color.pal'
  # cbrange[-6:-2]
  unset key
  set style fill solid
  
  set ylabel "Snapshot/Time"
  # set yrange [0.5:n+0.5]
  # set ytics 1    # manage it well in order to avoid black lines at the 
  borders
  set xrange[0:2021]
  set yrange[0:8]
  # This functions gives the name of the snapshot file
  snapshot(i) = sprintf("snapshot_%04d.dat", i)
  plot for [i=1:n] './snapshots/'. snapshot(n+1-i) using 1:2:3 with boxes 
       linecolor palette
  </code></pre>
  
  <p>where the result is: <a href="https://i.stack.imgur.com/mZ6CH.png" rel="nofollow noreferrer">picture</a></p>
  
  <p>The snapshots look like </p>
  
  <pre><code>snapshot_0001.dat  snapshot_0002.dat  snapshot_0003.dat  
     1 1.0 0.0            1 1.0 0.0            1 1.0 0.0           
     2 1.5 0.0            2 1.5 0.0            2 1.5 0.0           
     3 2.0 0.5            3 2.0 0.7            3 2.0 0.7            
     4 2.5 1.0            4 2.5 1.5            4 2.5 1.5           
     5 3.0 0.5            5 3.0 0.7            5 3.0 1.1           
     6 3.5 0.0            6 3.5 0.0            6 3.5 0.7           
     7 4.0 0.0            7 4.0 0.0            7 4.0 0.0          
     8 4.5 0.0            8 4.5 0.0            8 4.5 0.0           
     9 5.0 0.0            9 5.0 0.0            9 5.0 0.0            
  </code></pre>
  
  <p>Of course these are fake data since the original files are too heavy to add to this post. I would like to add  horizontal lines at particular times (axis labelled SnapShot/TIME).</p>
  
  <p>Is someone can help me with this issue?</p>