x1   x2   i   y
1   1   1   10.6
1   1   2    9.8
1   1   3   10.1
1   1   4   10.7
1   2   1   18.3
1   2   2   20.1
1   2   3   19.6
1   2   4   20.2
2   1   1   11.3
2   1   2   10.9
2   1   3   10.5
2   1   4   10.2
2   2   1   34.2
2   2   2   33.8
2   2   3   34.6
2   2   4   33.5