I'm looking to do something similar, extract a daily dump from some of the dashboards to import into another presentation tool for consumption by my teammates, the Capacity Planning team and management in general.
So far, no luck, and the API docs are - to be frank - unimpressive. It's missing some query parameters as well as lacking format specifications for parameter values that need to conform to a standard, again especially query-related parameters.
The current docs need to be vetted by a team that has never seen them before and has no inherent knowledge of the inner workings of the tool. The current docs make too many assumptions about what the user knows about HCM.