The best you can do is export from a single component (i.e. table), take a screenshot of the dashboard or export the dashboard to load into another banana instance. The reason why you can't do an offline dashboard is because you would need the entire index. Dashboards typically contain summarized data and/or a subset of detailed records. In order for the dashboard to remain interactive (search, filter, faceting, etc) you would need the entire data set offline because it does all of the counts/aggregations on the fly.