The near uniformity of the slope could be explained by a constant President-Only/Mixed vote for Trump. If that number is assumed to be a constant 50% and orthogonal to the Straight Ballot share, then you'll see a constant 10% decrease for every 10% increase of Straight Ballot R share above 50%... linearly. Also, at below 20%, likely because of small N of straight R ballots.
I don't think that's the right take. Assuming that the President Only voters voted at an even 50%, irrespective of straight-line outcome, a downward sloping graph is precisely what you'd expect.
The near uniformity of the slope could be explained by a constant President-Only/Mixed vote for Trump. If that number is assumed to be a constant 50% and orthogonal to the Straight Ballot share, then you'll see a constant 10% decrease for every 10% increase of Straight Ballot R share above 50%... linearly. Also, at below 20%, likely because of small N of straight R ballots.