Making a graph from data of one out every n lines

I have data that comes in batches of N values ("Y" values if you like) for a given index (or "X" value if you like). How to make N separate series in my graph for each entry of a given index ?

For example, if N = 3, I could have :

0   |   210   |   400   |   891   |   761   |   491   |   962   |   182   |   892   |   163   |   753   |   73   |   874   |   354   |   284   |   96... |   ...

And taking every third line I would want a graph of the following series :

0   |   891   |   962   |   163   |   874   |   96

Same for every second line and every first, etc.