This is a statistical graph (of the difference in values), not a plot of raw figures.

It’s not my preferred view any more than I find heat-maps particularly useful (my mind just doesn’t work that way) but there’s a long history of presenting that kind of analysis in that way.

It’s as intuitive as banks crediting you with a negative balance when you take out a loan, rather than with a positive sum of money, but it’s nothing new — you’re just stuck in the 5th Century when it comes to statistical analysis, I’m afraid ; )