title: All points y value undefined! in gnuplot
body: <p>I'm getting this error:</p>
  
  <p><code>line 0:all points y value undefined!</code></p>
  
  <p>My code for gnuplot is:</p>
  
  <pre><code>set terminal png 8;
  set output "skript01_31/image10.png"; 
  set timefmt x "%Y-%m-%d %H:%M:%S";
  set xdata time; 
  set format x "%H:%M"; 
  set xlabel "Cas"; 
  set ylabel "Hodnota"; 
  set yrange [:]; 
  set xrange ["2009-05-11 07:30:00":"2009-06-11 00:34:00"]; 
  set title "skript01"; 
  set grid;
  plot 'skript01_31/gnuplot_file' using 1:3 with points pt 7 ps 1 title "./skript01.sh";
  </code></pre>
  
  <p>My data file:</p>
  
  <pre><code>[2009-05-11 07:30:00] 0
  [2009-05-11 07:31:00] 0.00999983333416666468
  [2009-05-11 07:32:00] 0.01999866669333307936
  [2009-05-11 07:33:00] 0.02999550020249566076
  [2009-05-11 07:34:00] 0.03998933418663415945
  [2009-06-11 00:34:00] 0.05
  </code></pre>
  
  <p>What I'm doing wrong? </p>
  
  <p>Thanks for answer.</p>