function yi = SplineC1(x, y, dy, xi)
  for i = 2 : length(x)
    if xi <= x(i)
      j = i - 1;
      break;
    endif
  endfor
  i = j;
  h(i) = x(i + 1) - x(i);
  t = (xi - x(j)) / (h(j));
  yi = y(j)*(1-t)^3+(3*y(j)+h(j)*dy(j))*t*(1-t)^2+(3*y(j+1)-h(j)*dy(j+1))*t^2*(1-t)+y(j+1)*t^3;
 endfunction