The line represents each dollar of GDP added for each incremental dollar of Federal Debt.
I would suggest that someone look into why the velocity of debt is now running into the law of diminishing returns. The big slide started with Reagonomics and the 'supply side' theory based on the second chart.
It might have something to do with a corrupt financial system, the myth of efficient markets and globalization, tax cuts for the wealthy and unfunded wars, and the largely stagnating median wage.
However one wishes to slice it, it might be difficult to make up in volume what is taken away by a crippled financial system that keeps sweeping the productivity gains and national wealth up to the one percent where it is used for largely non-productive monopolization of resources, political corruption, unfunded endless wars, and fraud.