Same problem I am having right now. We just want to get the monthly totals for the Gross Sales, Discounts, and Returns.
Currently we have to use the Admin API to fetch individual Order data, then aggregate the numbers from there.
It feels like doing order-based fetching is a waste of resources on Shopify’s end (and the API consumer) since we only need the totals. In the Admin Analytics UI, you can query using ShopifyQL, but from searching I don’t think this is possible using the Admin GraphQL API.