Shopify natively only shows sales at product and store levels leaving collections and especially product categories in the dark.
I got frustrated about that and built a zero setup app that is still free to use. There are some other cool analytics such as sales per unique products in a category/collection and you can drill up and down between categories with just one click without any setup at all.
Check out portfolytics.app and the promo video there, and if you got interested at all, drop me a line here or on the site and I’ll give you a link. Or become a beta tester and get 2 years for free. win-win 
Best,
Ville, founder of Portfolytics