title: gnuplot heat map color range
body: <p>I have some X Y Z data in a file and I'm using gnuplot to display it.
  I am creating a heat map, ie. a 2D plot where the Z value is presented using color.
  Right now I'm using the following script:</p>
  
  <pre><code>set palette defined (0 "blue", 1 "red")
  plot "xyz.dat" u $1:$2:$3 w image
  </code></pre>
  
  <p>My problem is, gnuplot ignores the 0 and 1 in the palette definition. It uses the colors specified, but rescales according to the minimal and maximal Z value in the file.
  This makes it hard to visually compare different plots whose z range is different.</p>
  
  <p>How do I tell gnuplot to stop rescaling the z color range?</p>