Your plot has 0-100% on the X axis, but only 0-10% on the Y axis. Also if you assume an equal % of Republican share and Trump share % then it would be a straight line at y = 0. Your data itself has a downward trend in that you start with 10% more Trump share on x = 0, and end with 5% less Trump share linearly at X = 1
Remember what Dr. Shiva's y-axis is: the Trump vote pct minus the Repub straight ticket vote pct.
If the Dem vote pct is very high, Trump will still get a certain pct of those Dem votes and, thus, outperform Republicans. If the Repub vote is high, Trump will get a certain pct of those Repub votes, but not enough Dem votes remain for him to outperform the Repubs.
In the analysis Trumps share of total votes doesn't include the votes from republican only ballots, its just the % of people that didnt vote straight party but did vote Trump. the % that voted dem wont effect either column.
Your plot has 0-100% on the X axis, but only 0-10% on the Y axis. Also if you assume an equal % of Republican share and Trump share % then it would be a straight line at y = 0. Your data itself has a downward trend in that you start with 10% more Trump share on x = 0, and end with 5% less Trump share linearly at X = 1
Remember what Dr. Shiva's y-axis is: the Trump vote pct minus the Repub straight ticket vote pct.
If the Dem vote pct is very high, Trump will still get a certain pct of those Dem votes and, thus, outperform Republicans. If the Repub vote is high, Trump will get a certain pct of those Repub votes, but not enough Dem votes remain for him to outperform the Repubs.
In the analysis Trumps share of total votes doesn't include the votes from republican only ballots, its just the % of people that didnt vote straight party but did vote Trump. the % that voted dem wont effect either column.
The negative slope is still present if you only count Trump's share of the Dem vote. Try it yourself!