title: how to plot file contains many columns and column header using gnuplot
body: <p>I have some data file like this:</p>
  
  <pre><code>Id a1 a2 a3
  1  1  2  3 
  2  2  3  4 
  3  2  3  4 
  </code></pre>
  
  <p>But I don't know the exact number of column, but I can get it into variable with shell. And I want to plot the data file, the first column as the x-axis and the others as the y-axis in one picture and the column header as the title for line.like this:</p>
  
  <p><img src="https://i.stack.imgur.com/rpowt.png" alt="enter image description here"></p>
  
  <p>How to plot in gnuplot? Many thanks</p>