I’m not a Dr.!

I’d say you’re correct, there is a family of horizontal lines — but the only one that would travel all the way across the graph is when the intercept is 0.

Based off of Shiva’s plotted metric and the constraints that % values fall in the [0,100] interval, the only places points can be are in this region.

Also, the greater point: any inference you can make based off Shiva’s wonky metric, you can also make based off of a straight correlation between split-ticket votes and straight ticket votes. The fact that he continues using such a nonsensical metric tells me Shiva is not in fact arguing in good faith and is instead trying to obscure what’s going on.