Source: The White House Office of Management and Budget
Latest Data: 2021

This line item comes from the White House Office of Management and Budget, which publishes "The President's Budget" annually, a document which includes historical tables. The table reporting Federal Debt Held by the Public -- Total is titled "Federal Debt at the End of the Year."  

Source:  The White House Office of Budget and Management Historical Tables, Table 7.1—Federal Debt at the End of Year: 1940–2024