x=[-2 0 2 3 4 5];
y=[4 0 -4 -30 -40 -30];
plot(x,y,'sr')
hold on
p=polyfit(x,y,5)

p =

   -0.2286    2.5333   -6.2286  -10.1333   26.5714    0.0000

xx=-2:0.1:5;
yy=polyval(p,xx);
plot(xx,yy,'b')
ys=spline(x,y,xx);
plot(xx,ys,'g')