Hm, not sure I understand you.

Shiva is plotting % straight-ticket republican votes on the X axis, so let's call that variable X.

On the Y axis, he's plotting % split-ticket trump votes - X. Let's call % split-ticket trump votes Z.

Thus:

Y = Z - X,

or as I pose it:

Y = -X + Z, where Z is a random variable representing how % split-ticket trump votes are distributed.

EDIT: Worth emphasizing, Z is not a constant. It’s a Random Variable, which has an important definition here.