Hi, you can actually use the :groupby feature with external data source but you have to do the API call to get a list of results you can then group by month if you have a date field and aggregate.
e.g. with an API call to the Algolia API with a list of hits:
Just make the api call to get the data, but make sure you make the call type as DATA not action. it will give you a content type to choose in elements and you can use get external data to call that api call and get the data. then use grouped by to group them by a field.