OptionalgetOptionalgetRead a closed-bar snapshot without initiating a network request.
OptionalsubscribeOptionalopts: BarSubscriptionOptionsOptionalsubscribeopts.depthLevel requests a book depth (broker-dependent: 5/20/30/50).
Named on the interface so a caller holding a DataFeed can ask for one;
an implementation is free to ignore it and send the broker's default.
Optionalopts: { depthLevel?: number }
Broker-agnostic market-data source. The chart depends only on this.
subscribeBarsis optional: a history-only feed (e.g.OpenAlgoDataFeed) omits it, while a live feed (OpenAlgoLiveDataFeed, or your own) implements it.