Why did the slope go downwards? Because once you subtract the straight ticket voters (horizontal axis) the remaining voters (vertical axis) are more likely to vote for the opposition. That’s true for both parties.
“the remaining voters (vertical axis) are more likely to vote for the opposition.”
The analysis on this and it’s been extensive shows the mail in ballots that came in for Biden in Wisconsin, Pennsylvania and Georgia far exceeded the percentage of votes he could be expected to win at that stage.
Remember, this was happening in the dead of night. Those tallies are suspect for that reason alone.
But worse, the percentages were coming in at the greatest margin anywhere at any time across the tabulations of that state’s election.
It reeks and badly I might add.