function yi = SplineC1(x, y, dy, xi)
	n = length(x);
	for i=1:n-1
	a=y(i);
	d=y(i+1);
	h=x(i+1)-x(i);
	b=y(i) + h/3*dy(i);
	c=y(i+1) - h/3*dy(i+1);
	t=(xi-x(i))/h;
	if (xi >= x(i) && xi <= x(i+1))
	yi =a*(1-t)^3+3*b*t*(1-t)^2+3*c*t^2*(1-t)+d*t^3;
	endif
	endfor
endfunction