title: Gnuplot using column header
body: <p>In gnuplot it is possible to use the column name (header) instead of the column numbers.</p>
  <p>Let's assume the following datafile 'data.csv':</p>
  <pre><code>Time,X1,X2
  3600,1,2
  3601,3,4
  3602,5,6
  </code></pre>
  <p>Now the following two ways are possible:</p>
  <pre><code>plot 'data.csv' using 1:2 title 'Param 1', \
       'data.csv' using 1:3 title 'Param 2'
  
  plot 'data.csv' using &quot;Time&quot;:&quot;X1&quot; title 'Param 1', \
       'data.csv' using &quot;Time&quot;:&quot;X2&quot; title 'Param 2'
  </code></pre>
  <p>But if I want to perform some calculation before plotting - for example to plot the column 'Time' in hours instead of seconds, this is possible with the first way:</p>
  <pre><code>plot 'data.csv' using ($1/3600):2 title 'Param 1', \
       'data.csv' using ($1/3600):3 title 'Param 2'
  </code></pre>
  <p>But the following is not possible:</p>
  <pre><code>plot 'data.csv' using (&quot;Time&quot;/3600):2 title 'Param 1', \
       'data.csv' using (&quot;Time&quot;/3600):3 title 'Param 2'
  </code></pre>
  <p>How can I perform calculations on columns specified by column header?</p>