Find the missing values using Lagrange polynomials

First of all , I am finding three missing values in my data. Second, I need to find the value of the mean carbon dioxide measurement on specific data. I put lagrange(x,pointx,pointy), but it just keep come up a wrong number(386.9400).

clc;
clear;
load C02Data.mat;
x = C02Data(:,2);
pointx = C02Data(:,2);
pointy = C02Data(:,3);
lagrange(x,pointx,pointy)

function y=lagrange(x,pointx,pointy)
  n=size(pointx,2);
  L=ones(n,size(x,2));
  if (size(pointx,2)~=size(pointy,2))
     fprintf(1,'\nERROR!\nPOINTX and POINTY must have the same number of elements\n');
     y=NaN;
  else
     for i=1:n
        for j=1:n
           if (i~=j)
              L(i,:)=L(i,:).*(x-pointx(j))/(pointx(i)-pointx(j));
           end
        end
     end
     y=0;
     for i=1:n
        y=y+pointy(i)*L(i,:);
     end
  end