Update (July 15, 2018):

• Shopify has recently updated their API to now expose additional discount information on the line item level. Please use the Lineitem Discount Allocations fields instead for web orders.

Line item discounts can only be applied via the Shopify POS or when creating an order manually via the Shopify Admin.

So for regular web orders, this data will always be 0, even if the discount code that was applied to the order is set up only for specific products.

In the Shopify Admin, you can apply a discount per line item when you create an order manually. All you need to do is click the price link for each line item as shown in the screenshot below:

There's definitely quite a bit of confusion around this and, unfortunately, there is no workaround that we're aware of to figure this out for online orders.

More Info: https://ecommerce.shopify.com/c/api-announcements/...