cubicFit.m

% Interactive script to fit a cubic to data points

clf
hold on
axis([0 100 0 100]);

diff = 10;
xold = 68;
i = 0;
xp = zeros(1);       % data points
yp = zeros(1);

while diff > 2
  [a b] = ginput(1);
  diff = abs(a - xold);
  if diff > 2
    i = i + 1;
    xp(i) = a;
    yp(i) = b;
    xold = a;
    plot(a, b, 'ok')
  end
end

p = polyfit(xp, yp, 3 );
x = 0:0.1:xp(length(xp));
y= p(1)*x.^3 + p(2)*x.^2 + p(3)*x + p(4);
plot(x,y), title( 'cubic polynomial fit'), ...
   ylabel('y(x)'), xlabel('x')
hold off