title: How to plot points specified by ID column only with linespoints and multiple plots using gnuplot?
body: <p>Say I have two files, each has 3 columns<br>
  file1:  </p>
  
  <pre><code>ID X Y  
  10 0.1 some data as X  
  20 0.2  
  30 0.3  
  ... ...  
  120 0.5  
  </code></pre>
  
  <p>file2:  </p>
  
  <pre><code>ID    X    Y  
  15   0.1   some data as X  
  30   0.2  
  45   0.3  
  60   0.4  
  ...  ...  
  120  0.6
  </code></pre>
  
  <p>I am doing   </p>
  
  <pre><code>plot \  
  "file1" using 2:3 w linespoints lt 1 dt 1 lw 1 lc 1 title "file1",\  
  "file2" using 2:3 w linespoints lt 1 dt 1 lw 1 lc 2 title "file2"  
  </code></pre>
  
  <p>which shows every point on the file.  </p>
  
  <p>If I only want points which its Row ID(first column) is 30, 60 ,90 ,120<br>
  How should I do? Thank you.  </p>
  
  <p>*In the actually case, I need to plot 12 file in one plot and each of them have 10000 rows but I only want to show 6 points.  </p>