That's how it looks, yes, but that doesn't explain why it's called zero-sum. If you multiplied all the scores by some factor so that their sum was zero, then they would all be zero, and the graph would be rather dull.
I would have guessed, given no information but the name, that a zero-sum graph would be one where you added the same thing to each of the scores so that their sum was zero (you would in fact have to subtract the mean of the scores) - but that's not what is being done here.
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