title: gnuplot warning: Skipping data file with no valid points, using a very simple file.dat
body: <p>i'd like to understand the use of "index" command to plot only some groups of datas or for example to plot every group but with different titles etc..</p>
  
  <p>i started with an easy example: in the file prova.dat i have the following text (i've already checked that there are not white records a the end of the lines):</p>
  
  <pre><code>1 1
  2 2
  3 3
  4 4
  
  1 2
  2 3
  3 4
  4 5
  </code></pre>
  
  <p>then i use this code to plot data from 'prova.dat':</p>
  
  <pre><code> set term postscript  eps enhanced font 'Times-Roman,24'
   set output 'prova.eps'
   set key right top
   set xrange [1:5]
   set yrange [1:5]
   plot 'prova.dat' index 0 w lines title columnheader(1), \
        'prova.dat' index 1 w lines title columnheader(1) 
  </code></pre>
  
  <p>but i get the following warning:</p>
  
  <pre><code>warning: Skipping data file with no valid points
  </code></pre>
  
  <p>thus in the .eps produced i have no plots but just the axis (correctly formatted).</p>
  
  <p>i've read also other posts but i can't find a solution.</p>
  
  <p>thank you in advance!</p>