title: Gnuplot : filledcurve between smoothed curve
body: <p>I have two sets of points: "test1.dat" and "test2.dat", they do not share same X-values.</p>
  
  <p>I want to draw two smooth lines first due to the data is noisy, and 
  then draw a filledcurve between the smooth lines.</p>
  
  <p>I've read the tutorial, and cannot find the answer.</p>