Correct my if I'm wrong but they appear to have made a huge mistake.

They plot T - R vs R, where T is the individual Trump vote % and R is the straight republican vote %.

Consider what the chart would look like if they plotted -R vs R: It would look like a line going down and to the right at a 45 degree angle, so that when R is 60%, -R is at -60%.

They are plotting T - R vs R, so imagine the downard sloping -R vs R line and add the Trump vote % to it. A simple assumption is that T will be correlated with R, so you would expect that we will still see a sloping line, which is what we see.

Correct my if I'm wrong but they appear to have made a huge mistake.

They plot T - R vs R, where T is the individual Trump vote % and R is the straight republican vote %.

Consider what the chart would look like if they plotted -R vs R: It would look like a line going down and to the right at a 45 degree angle, so that when R is -60^, -R is at -60%.

They are plotting T - R vs R, so imagine the downard sloping -R vs R line and add the Trump vote % to it. A simple assumption is that T will be correlated with R, so you would expect that we will still see a sloping line, which is what we see.