title: gnuplot date output in X doesn't display right
body: <p>Im parsing linux sar output and i have dat file which looks like this :</p>
  
  <pre><code>07:09:49      CPU      %usr     %nice      %sys   %iowait    %steal      %irq     %soft    %guest     %idle
  07:09:51      all      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00    100.00
  07:09:53      all     11.82      0.00      0.13      0.00      0.00      0.00      0.00      0.00     88.05
  07:09:55      all     53.99      0.00      0.63      0.00      0.13      0.00      0.13      0.00     45.12
  07:09:57      all     55.18      0.00      0.25      0.00      0.00      0.00      0.00      0.00     44.57
  07:09:59      all     66.58      0.00      0.51      0.00      0.00      0.00      0.13      0.00     32.78
  07:10:01      all     71.90      0.00      0.63      0.13      0.00      0.00      0.13      0.00     27.22
  07:10:03      all     70.24      0.00      0.63      0.00      0.00      0.00      0.13      0.00     29.00
  07:10:05      all     55.39      0.00      0.63      0.00      0.00      0.00      0.13      0.00     43.85
  07:10:07      all     72.90      0.00      0.38      0.00      0.00      0.00      0.00      0.00     26.73
  07:10:09      all     60.96      0.00      0.38      0.00      0.13      0.00      0.13      0.00     38.40
  07:10:11      all     76.60      0.00      0.63      0.00      0.00      0.00      0.13      0.00     22.65
  07:10:13      all     53.87      0.00      0.76      0.00      0.00      0.00      0.13      0.00     45.25
  07:10:15      all     46.73      0.00      0.63      0.00      0.00      0.00      0.00      0.00     52.64
  07:10:17      all     56.37      0.00      0.50      0.00      0.00      0.00      0.13      0.00     43.00
  07:10:19      all     58.15      0.00      0.63      0.00      0.00      0.00      0.13      0.00     41.09
  07:10:21      all     61.26      0.00      0.75      0.00      0.00      0.00      0.13      0.00     37.86
  07:10:23      all     51.50      0.00      0.75      0.12      0.12      0.00      0.25      0.00     47.2
  
  
  set title ' CPU usage'
  set xdata time
  set timefmt '%H:%M:%S'
  set xlabel 'time'
  set ylabel 'CPU Usage'
  set style data lines
  plot 'filename.dat'  using 1:3 title '0.6'
  pause -1
  </code></pre>
  
  <p>the out put in the X data is not related to the time presented in the file 
  <a href="https://i.stack.imgur.com/daUGJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/daUGJ.png" alt="enter image description here"></a></p>