I did not flip it around.
Y = % split-ticket voters - % straight-ticket voters
Y = -X + % split-ticket voters; where X is % straight-ticket voters.
Those two are the same thing.
A - B is the same as (-B + A), no?
I changed the order to get it to y=mx+b form and make it easier for the reader to make the leap. It's still the same thing.