% Fits a line to a set of of points. %
function line_fit(x, y) =
let 
  n    = float(#x);
  xa   = sum(x)/n;
  ya   = sum(y)/n;
  Stt  = sum({(x - xa)^2: x});
  b    = sum({(x - xa)*y: x; y})/Stt;
  a    = ya - xa*b;
  chi2 = sum({(y-a-b*x)^2: x; y});
  siga = sqrt((1.0/n + xa^2/Stt)*chi2/n);
  sigb = sqrt((1.0/Stt)*chi2/n)
in 
  (a, b, siga, sigb) $