title: Draw a line in a pm3d map
body: <p>I have a<img src="https://i.stack.imgur.com/sBJXX.png" alt="density map"> and I want to overplot on it a <img src="https://i.stack.imgur.com/GQjBJ.png" alt="line">.</p>
  
  <p>I define the line as a 3d line with constant height and I thought that with this method I have the possibility to overplot them one to another but, unfortunately, I failed.
  I realized in fact that the density map routine in gnuplot take the points of the line too.</p>
  
  <p>This is my script:</p>
  
  <pre><code>reset
  
  #set terminal pngcairo size 800,500 enhanced font 'Verdana,14'
  #set output "map.png"
  set title 'Map'
  
  
  m_star=1.054
  m_1=0.0093866
  m_3=0.0078635
  
  fact1 = (m_star+m_1)**0.5
  fact2 = (m_star+m_3)**0.5
  
  set pm3d map interpolate 1,1
  set xrange [50.22:52.27]
  set yrange [30.29:31.17]
  
  splot "file.txt" u (fact1)*(($1)**(-1.5)):(fact2)*(($2)**(-1.5)):6 notitle ,\
        "line.txt" notitle 
  </code></pre>
  
  <p>And here the two files: <a href="http://www.filedropper.com/line_1" rel="nofollow noreferrer">line</a> and <a href="http://www.filedropper.com/file_282" rel="nofollow noreferrer">density map</a>.</p>
  
  <p>It is possible to overplot them, in gnuplot? </p>
  
  <p>Thanks a lot for your help !</p>