title: Gnuplot (fitting of ellipse)
body: <p>I'm trying to fit an ellipse using gnuplot.</p>
  
  <p>However, when I defined my equation</p>
  
  <pre><code>f(x,y) = a*x*x + b*x*y + c*y*y + d*x + e*y + f
  </code></pre>
  
  <p>and then proceed with the command plot:</p>
  
  <pre><code>plot f(x,y) lw 3 lc rgb 'black', 'text.dat' w l lc rgb 'black'
  </code></pre>
  
  <p>I get <code>undefined variable: y</code>. How can I fix it?
  <code>text.dat</code> has 3 columns: x, y, and z coordinates.</p>