x || -2 | -1 |  1 |  2 |  3
           ---++----+----+----+----+----
	    y ||  2 |  1 | -1 |  0 |  2

>> x = [-2 -1 1 2 3]

x =

    -2    -1     1     2     3

>> y = [2 1 -1 0 2]

y =

     2     1    -1     0     2

>> c = polyfit(x,y,2)

c =

    0.4529   -0.5503   -0.5909

% The vector c contains the a, b and c of the quadratic
% y = a*x^2 + b*x + c in its entries.
% Now we plot the results, first defining the plot range
r = -2:0.1:3;
f = polyval(c,r);
plot(r,f)
hold on
plot(x,y,'r+')