Because the y-axis is Trump vote minus Republican vote and because the x-axis is Republican vote, there will always be a negative slope if you assume the Trump vote is less than 100% of the Republican vote. It would be a positive slope if the Trump vote is greater than 100% of the Republican vote. The only time it will be a zero slope line is of the Trump vote is exactly the same as the Republican vote.

It works like this:

1. Suppose that Trump got 90% of the Republican vote.
2. Suppose that a particular precinct is 100% Republican.
3. In that precinct, Trump would get 90% of the vote.
4. This would be a margin of -10% for Trump.
5. Now suppose a precinct is 50% Republican.
6. In that precinct, Trump would get 90% of the Republican vote, but that comes out to only 45% of the total vote.
7. This would be a margin of -5%.

See the pattern? A negative slope is baked into the y-axis.