# A quick math question for NX curve

My text book says as if this $$NX$$ and $$S-I$$ graph can explain everything, like even when the economy experiences trade deficit. But shouldn't $$S-I$$ be a minus value when it's in a deficit? Then how could one adjust this graph for that? Could it be that the 0 point is placed between $$NX_2$$ and $$NX_1$$? (assuming $$S_2$$ < $$I$$ < $$S_1$$)