As far as I know, the only indexing services which track citation across journals and data repositories are accessed for a fee. Are there any free-to-use databases of article or data citation indices ?
I work for a biomedical literature database - Europe PMC, which makes citation data freely available. You can access it programmatically, via the articles API (https://europepmc.org/RestfulWebService#cites). This resource also tracks data citations for over 20 different public resources: https://europepmc.org/Help#trackdatacit