The graph uses data from the NSF's Survey of Graduate Students and Postdoctorates in Science and Engineering (catchy name, that!) which does not suffer from the limitation you describe. The SGSPSE does miss people who are not in academic institutions (e.g. the 3000+ postdocs at NIH and maybe a few thousand others scattered around national labs plus a handful in industry), and it's only a headcount. I suspect you may be confusing this survey with the Survey of Doctorate Recipients which does miss out on postdocs who earned their PhDs outside the US.
Hi Flygal--
The graph uses data from the NSF's Survey of Graduate Students and Postdoctorates in Science and Engineering (catchy name, that!) which does not suffer from the limitation you describe. The SGSPSE does miss people who are not in academic institutions (e.g. the 3000+ postdocs at NIH and maybe a few thousand others scattered around national labs plus a handful in industry), and it's only a headcount. I suspect you may be confusing this survey with the Survey of Doctorate Recipients which does miss out on postdocs who earned their PhDs outside the US.
I'm glad you found it interesting!
Geoff