(x_i,y_i) : x_i<x_{i+1}, i=1~n
f_2(x)=((x_2-x)y_1+(x-x_1)y_2)/(x_2-x_1)
f_i(x)=f_{i-1}(x)+(1/2)(x-x_{i-1}+|x-x_{i-1}|)(y_i-f_{i-1}(x_i))/(x_i-x_{i-1})