title: Fit with conditional statement returning NaN
body: <p>I have the following dataset</p>
  
  <pre><code>4456.5  140.5   1
  4371.4  142.1   1
  4389.8  150.5   1
  4716.9  306.3   4
  7505.4  1117.6  4
  13517.3 2298.2  2
  13961.3 2358.3  2
  13994.7 2444.3  2
  </code></pre>
  
  <p>and I want to fit only the points with a specific value in the third column.
  With <code>plot</code> I'd use the conditional <code>using 1:($3==1?$2:NaN)</code> but</p>
  
  <pre><code>LS(x) = a*x**2 + b*x + c
  a=0.00001; b1=0.; c=0. 
  fit LS(x) IFN u 1:($3==1?$2:NaN) via a,b,c
  </code></pre>
  
  <p>doesn't work because the <code>NaN</code> values are fitted as well and I get </p>
  
  <pre><code>&gt; print a1,b1,c1
  NaN NaN 0.0001
  </code></pre>
  
  <p>Is there a way to fit a conditional subset without having to re-write the input file?</p>