Hi,
Try below (it uses least squares):
For lagrange you can look at that link:
x1 = 1;
y1 = 3.5;
x2 = 2;
y2 = 14/3;
x3 = 3;
y3 = 14;
x4 = 4;
y4 = 28;
x1Array = [x1,x2,x3,x4];
y1Array = [y1,y2,y3,y4];
n = 1; % polynomial degree (you can change it as you wish)
p = polyfit(x1Array,y1Array,n); % p is coefficient of your polynomial: P(X) = P(1)*X^N + P(2)*X^(N-1) +...+ P(N)*X + P(N+1) descending order.
newY = polyval(p,x1Array); % function results
plot(x1Array,y1Array, 'bo',x1Array,newY,'-r');
grid;
legend('Data','Fitted Data');
