This is just a model to illustrate why a negative slope is expected.
Trump won a certain pct of Dems and a certain pct of Repubs. When the overall Dem pct is high, he's going to outperform Repubs. When the overall Repub vote is high, he's going to underperform Repubs. This is just a mathematical certainty.
A negative slope is expected with random values, the point is the correlation between Republican straight party vote and Trump single vote is not random, you would expect it to be a fairly constant ratio, aka a straight line.
My argument here is that you wouldn't expect a straight line. It's a bit counterintuitive until you start playing with the numbers yourself. The problem is that the y-axis represents different in pct not a pct. So as the pct on the x-axis increases, the difference between the pcts will increase.
This is just a model to illustrate why a negative slope is expected.
Trump won a certain pct of Dems and a certain pct of Repubs. When the overall Dem pct is high, he's going to outperform Repubs. When the overall Repub vote is high, he's going to underperform Repubs. This is just a mathematical certainty.
A negative slope is expected with random values, the point is the correlation between Republican straight party vote and Trump single vote is not random, you would expect it to be a fairly constant ratio, aka a straight line.
My argument here is that you wouldn't expect a straight line. It's a bit counterintuitive until you start playing with the numbers yourself. The problem is that the y-axis represents different in pct not a pct. So as the pct on the x-axis increases, the difference between the pcts will increase.
The real world correlation says that as the X axis % increases, so will the Trump vote on the Y axis, thus negating it.