I'll try my best:
When you know the number of cells in a lipo pack, you will also know the total voltage & the fully charged voltage of each cell- in this case 16.8v & 4.2v per cell. When you look at the graph, you can see where the big spikes in power happen ( massive current draw, in amps ), and at the same time on the graph, you can see how the total voltage drops. By comparing them together you can work out the voltage of each cell as it is heavily loaded ( by the current draw ) by dividing the total voltage at that point by the number of cells ( say, 13.19v / 4 = 3.29v per cell ).
You can also work out a fairly accurate C rating for the pack based on the current draw at that point in the graph too. In this case its 150amps ( the maximum current pulled during that run ) divided by the pack's rated capacity of 5amps ( 5000mah ), so 150 / 5 = 30C, since the C rating is just the rated capacity multiplied by how much current it can produce before the cell voltage drops below a certain level. In this case the 'certain level' is 3.3v per cell, an acceptable level in my book, though it would be nicer if it was a little higher ( its always nicer.... ).
Hope that makes a bit more sense.
