title: gnuplot: Plot and fit 2d function with two variables
body: <p>Is its possible in gnuplot to plot and fit a function that has two variables? For example, a physical function that depends on hight <code>h</code> and Temperature <code>T</code> where the <code>T</code> dependence should only be calculated but not plotted (for <code>f</code>, <code>h</code> and <code>T</code> experimental data exists):</p>
  
  <pre><code>f(h,T) = a * h * (1 + alpha * T) + f0
  </code></pre>
  
  <p>where <code>a</code> and <code>f0</code> are to be determined by the fit, <code>alpha</code> is known. In the end I need a plot with <code>f</code> on the y-axis and <code>h</code> on the x-axis. The whole <code>T</code> dependence should be taken care of in the fit, but I don't need it displayed with <code>splot</code>.</p>
  
  <p>The following is what I tried and failed. I assume because one can't set two dummy variables:</p>
  
  <pre><code>set term png;
  set output 'test.png';
  set dummy h;
  set dummy T;
  f(h,T) = a * h * (1 + alpha * T) + f0;
  fit f(h,T) 'data.txt' using 2:4:1 via a, f0;
  plot f(h,T);
  </code></pre>
  
  <p>gives <code>undefined variable: h</code>. Any ideas?</p>