title: Need to plot every n'th row in gnuplot
body: <p>I am using gnuplot and have some data that have more than 2 parameters. I don't want to use 3D graphs. So I will split the data up in several lines in the same plot. </p>
  
  <p>My data looks like this, in one case: </p>
  
  <pre><code>    #k   #N   #time   
  1   1   0.000556134 
  2   1   0.00099 
  4   1   0.00201011  
  8   1   0.00376214  
  16  1   0.00675843  
  1   2   0.000717646 
  2   2   0.000794106 
  4   2   0.0016033   
  8   2   0.0033602   
  16  2   0.00795338  
  1   4   0.000476448
  ...     ...     ... 
  </code></pre>
  
  <p>First is number of streams i use, second is buffer size and the last is time use on this opperation. </p>
  
  <p>I want one graph where Buffer size is X, time Y and there is one line for every "amount of streams used". My code up to now:</p>
  
  <pre><code>     reset
          set   autoscale # scale axes automatically
          unset log       # remove any log-scaling
          unset label     # remove any previous labels
          set xtic auto   # set xtics automatically
          set ytic auto   # set ytics automatically
          #set logscale x 2
          #set logscale y 2
          set rmargin 10
          set bmargin 5
          set tmargin 5 
          set key left top
          set title
          set term png giant size 1500, 800 crop
  
          set title "Adapter Stream Input" font "verdana, 20"
          set xlabel "Buffer Size" font "verdana, 20"
          set ylabel "Time in Seconds" font "verdana, 20"
  
          set key right bottom spacing 4 width 5 height 5 font "verdana, 15"
  
          set output "AdapterStreamInputTests.png"
  
          plot    "AdapterStreamInputTests.txt" using ($1 == 1 ? $2 : 1/0):($1 == 1 ? $3 : 1/0) title '1 Stream' with linespoints ls 1 linecolor rgb "blue", \
          "AdapterStreamInputTests.txt" using ($1 == 2 ? $2 : 1/0):($1 == 2 ? $3 : 1/0) title '2 Stream' with linespoints ls 1 linecolor rgb "red", \
          "AdapterStreamInputTests.txt" using ($1 == 4 ? $2 : 1/0):($1 == 4 ? $3 : 1/0) title '4 Stream' with linespoints ls 1 linecolor rgb "yellow", \
          "AdapterStreamInputTests.txt" using ($1 == 8 ? $2 : 1/0):($1 == 8 ? $3 : 1/0) title '8 Stream' with linespoints ls 1 linecolor rgb "brown", \
          "AdapterStreamInputTests.txt" using ($1 == 16 ? $2 : 1/0):($1 == 16 ? $3 : 1/0) title '16 Stream' with linespoints ls 1 linecolor rgb "pink"
  
  set output
  </code></pre>
  
  <p>My main problem now is that it plots everything correct but it wont make lines between the plots. I read from the <a href="http://www.gnuplot.info/docs_4.4/gnuplot.pdf" rel="nofollow">manual</a> under the command "using" chapter that:</p>
  
  <blockquote>
    <p>It should be noted that plot 'le', plot 'le' using 1:2, and plot
    'le' using ($1):($2) can be subtly dierent: 1) if le has some lines
    with one column and some with two, the rst will invent x values when
    they are missing, the second will quietly ignore the lines with one
    column, and the third will store an undened value for lines with one
    point (so that in a plot with lines, no line joins points across the
    bad point); 2) if a line contains text at the rst column, the rst
    will abort the plot on an error, but the second and third should
    quietly skip the garbage.</p>
  </blockquote>
  
  <p>I think I am in the third catagory and there is no lines because every plot have 4 invalid plots between them, but I cant find any way to fix it. </p>
  
  <p>I can't find a way to make it ignore invalid plots. And there does not seem to be any way of preprocessing that I was able to find, that could remove the unused plots. But I am probably missing something. </p>