Question

plot the relationship between national debt and lending rates

Answer 1

The difficulty is that there are many factors that affect bond yields – and government debt and the budget deficit is often an unimportant factor. Only in certain circumstances will levels of government borrowing have a major impact on bond yields.
The most important factors for determining bond yields are probably inflation, interest rates and the rate of economic growth.
Source:
https://www.economicshelp.org/blog/4966/debt/link-between-debt-and-bond-yields/