Point is that it seems by the above calculation that a change of increase-by-25% and a change of decrease-by-20% are opposite changes of equivalent magnitude as

Point is that it seems by the above calculation that a change of increase-by-25% and a change of decrease-by-20% are opposite changes of equivalent magnitude as they perfectly cancel each other out. This is why they are plotted equidistant from 0% on the original chart.