title: gnuplot draw two plots on the same graph with single column data
body: <p>There are two data files, say <strong>data1.txt</strong>:</p>
  
  <pre><code>31231
  32312
  32323
  32323
  </code></pre>
  
  <p><strong>data2.txt</strong>:</p>
  
  <pre><code>32323
  54223
  32456
  45321
  </code></pre>
  
  <p>I want to draw the two plots on the same graph, how can I use <code>gnuplot</code> to realize that? Thank yu very much.</p>