title: Gnuplot internal error : STRING operator applied to undefined or non-STRING variable
body: <p>I was trying to plot a 18 line graph with gnuplot, so I used the code below and it shows the non-string variable error.</p>
  <pre><code>plot for [num = 4560 : 5072 : 64] \
      &quot;graph_&quot;.num.&quot;.dat&quot; using 1:4 with lines lw 2 title &quot;&quot;.num.&quot;.fast&quot;, \
      &quot;graph_&quot;.num.&quot;.dat&quot; using 1:2 with lines lw 2 title &quot;&quot;.num.&quot;.slow&quot;
  replot
  </code></pre>
  <p>But when I only use one, it works perfectly.
  I thought it could be because of a maximum number of lines in the legend or something.
  How can I plot these 18 lines in the same graph?</p>