Index   X   Y   Z
1   A   A   1
2   A   B   5
3   A   C   2
4   A   D   42
5   A   E   10
6   A   F   2
7   A   G   0
8   A   H   3
9   A   I   0
10  A   J   8
11  A   K   19
12  A   L   3

Index   X   Y   Z   CumSum  Group
1   A   A   1   1   1
2   A   B   5   6   1
3   A   C   2   8   1
4   A   D   42  50  1
5   A   E   10  10  2
6   A   F   2   12  2
7   A   G   0   12  2
8   A   H   3   15  2
9   A   I   0   15  2
10  A   J   8   23  2
11  A   K   19  42  2
12  A   L   3   45  2