We use the js-buy-sdk with a custom react storefront, the add to cart in the shopify reports always shows 0 items have been added to cart. See screenshot:
We do however sell products, and they are showing in our reports for both 'reached checkouts' & 'sessions converted'. Only add to cart stays 0.
I suspect it's because the 'add to cart' action happens on our custom storefront, and for the checkout we route to shopify with the checkoutUrl from the js-buy-sdk.
How can we get the 'add to cart' metrics to show correctly in the shopify reports?
In an ideal situation, your addto-cart metrics is equal to the number of checkouts. In practices, add to cart session should be a lot more than successful checkouts. So mathematically your add-to-cart values should be No. of abandoned carts plus the actual successful checkouts. You can query abandoned carts via the abandoned cart API.
Hopeful this helps. DM me if you need more help since I'm working on a report app that could include additional metrics.