function [si]=cubplot(x,y)
[b,c,d]=cubspline(x,y)
xx=0:0.01:2*pi;
for j=1:length(xx)
    i=max(find(x<xx(j)))
    si(j)=y(i)+b(i)*(xx(j)-x(i))+c(i)*((xx(j)-x(i))^2)+d(i)*((xx(j)-x(i))^3);
end
figure
plot(xx,si)
end