Have you though about your SRAM? 72*72 = 5184, now you made them ints, so now thats 10368 of the 2K on an UNO or 4K on a Mega. 
Why do you need to record everything? Why not just draw the points as the data comes in? Of course you need two sets of coordinates, so that's where you have n_X/Ypoint(new) and o_X/Ypoint(old)
I'm sure you can work out the rest from there.
If you really need to record everything then look into other means of storage, like SD cards or maybe even use the Heap (limited there too).