I have many cols, same length and containing numeric data. I want to merge
them into one col. The merge should stack all of colB under colA, colC under
colB and so on.

Origainal data is:
A B C D ........
1 2 3 4 ........
. . . . ........
. . . . ........

After merge
A
1
..
..
2
..
..
3
..
..


1 2 3 4
1920 1.1 2.2 3.3 4.3
............................................... 5.7
1921 4.2 3.1 5.6 6.9
................................................ 6.8
1922 5.3 6.2 1.9 2.6
...............................................
...... .... .... .... ....
...............................................
2007 3.2 5.1 2.7 6.5 ................................5.9