The blue line is the expected interest payments that the US will need to make on its debt each year.
This projection doesn't take into consideration:
  • the quickly rising interest rates we're seeing today, or
  • a world where foreign governments are net sellers of US debt
Both of those could cause the blue lines to get exponentially bigger, leading to the US deficit spiralling out of control.
Currency devaluation seems like the most plausible way out.