title: gnuplot plotting a filled surface (polygon) using 3 points
body: <p>I am trying to draw a triangular surface. I am setting three corners of the surface as <code>[1,0,1]</code>, <code>[0,1,1]</code> and <code>[1,1,0]</code>. I am doing like this:</p>
  
  <pre><code>set view 57, 64
  $data &lt;&lt; EOD
  1.000 0.000 1.000
  0.000 1.000 1.000
  1.000 1.000 0.000
  1.000 0.000 1.000
  EOD
  set label 1 "C" font "Arial, 11" front at 1.100, 0.000, 1.100 
  set label 2 "A" font "Arial, 11" front at 0.000, 1.050, 1.050
  set label 3 "B" font "Arial, 11" front at 1.050, 1.050, 0.000
  splot "$data" with lines lw 3 lc rgb 'black', \
      "&lt; echo '1.000 0.000 1.000'" with points pt 7 lc rgb 'black', \
      "&lt; echo '0.000 1.000 1.000'" with points pt 7 lc rgb 'black', \
      "&lt; echo '1.000 1.000 0.000'" with points pt 7 lc rgb 'black'
  </code></pre>
  
  <p>and the plot I am getting is like this:
  <a href="https://i.stack.imgur.com/cJCFz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cJCFz.png" alt="enter image description here"></a></p>
  
  <p>Now how do I make the polygon grey or transparent grey?</p>