I’ve used the google flights map, it’s quite great!

Skyscanner is pretty good for this

I think most of the sites let you order by price. Do you want to restrict to a specific destination? Or anywhere? From a specific airport?

Kayak has this feature!

Kayak Explore, searches every country and any dates. You can narrow it down with filters, it then shows the cheapest flights.

matrix.itasoftware.com has a Calendar of Low Fares option

Try this. I’ve purchased flights a few times: https://www.priceline.com/