Here's my edited note about this, let me know if it addresses your concern, or if it doesn't:

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EDITED NOTE: Thank-you to the many respondents who have told me that the Y-axis quantity is explicitly the difference between straight-ticket voters for the Republican Party and votes for Trump. Straight-ticket voters have the option to manually choose individual candidates to override party defaults. Since a straight-ticket Republican vote with or without Trump is the equivalent to voting for all down-ballot Republicans, I’m dealing with the same quantity — I’ve just put it in more general terms. For the X-axis, Dr. Ayyadurai also uses this fraction straight-ticket Republican voters, whereas I keep it general and call it the “inferred % of Republican voters”. Again, these quantities are equivalent for the purposes of my analysis. As always, if you don’t agree and think it would change the results meaningfully, let me know.