The data com from Survey of Graduate Students and Postdoctorates in Science and Engineering (GSS) 2021. Only broad fields of postdocs are available, and mapping to the departments is impossible. Subfields are aggregated into three general categories, and the aggregate statistics is also given:
Two graphs for gender are given: one includes just the postdocs, and another includes postdocs and doctorate-holding nonfaculty researchers. Ethnicity data for doctorate-holding nonfaculty researchers is unavailable.
Also, note that the ethnicity is only specified for American citizens and permanent residents. The 3rd graph below reflects the data with non-residents omitted.