Ah, OK. In the Engine API, charts are children of sheets. To access them you either need to know the ID of the individual sheet objects directly, or navigate to them through the child list of a sheet. If there is a specific object you want to inspect, then I can recommend this tool for finding the ID of the object you are interested in:
That tool is also useful for navigating through the properties of the objects, which might also be of interest to you.
If, on the other hand, you don't know the ID up front, and want to navigate to it programatically, then I can recommend you to have a look at this example that traverses all objects of an app. It should give you some indications regarding which endpoints would be useful for you: