title: Including solid and dashed contours in gnuplot
body: <p>i have the following in a gnuplot script:</p>
  
  <pre><code>set pm3d
  unset surface
  set pm3d map
  set style line 1 lt 1 lc rgb "white"
  set style line 2 lt 1 lc rgb "white"
  set style line 3 lt 1 lc rgb "white"
  set style line 4 lt 1  lc rgb "red"
  set style line 5 lt 1  lc rgb "blue"
  set style line 6 lt 2  lc rgb "white"
  set style line 7 lt 2  lc rgb "white"
  set style line 8 lt 2  lc rgb "white"
  set style line 9 lt 2  lc rgb "white"
  set style increment userstyles
  set contour base
  set cntrparam levels 8
  set cntrparam levels discrete -8*0.0004946, -6*0.0004946, -4*0.0004946,     -2*0.0004946,    -2*0.0004946, 4*0.0004946, 6*0.0004946, 8*0.0004946
  set nokey
  splot '/data/ltl21/Data/PDB/HDFNI/BlindSurvey/imageresults/spectrum/HDF1map:189.20135-62.20442' u 1:2:3
  </code></pre>
  
  <p>Such that, i would like to have dashed contours for negative values, and solid contours for positive values.  I can change the colour of contours with the above fine, though i want them to all be white, but it doesn't seem to use lt at all, it just picks its own line style and uses that..</p>
  
  <p>Does anyone know how to specify the linetype for individual contours?</p>
  
  <p>Cheers</p>