The chart is listed as being getting the data from the US Dept of the Teasury.
Public held debt is an accepted measure of debt for the US government. People have used it to hammer Obama for a dramatic increase in it (as SS was either in a deficit or just slightly positive. Conservatives have used to to defend GWB buy stating the debt created under his admin was lower then it actually was
What it does not due is tell the full story of US government debt, it just tells one aspect of it