I'd like to find a tool where I can ask prices without any specific dates but specific days. Like "I want to go there between Friday to Sunday" and it will show me all the prices for the month. Google flight (and others I'm sure) offer "graph price" but it's also showing me flights for Wednesday to Friday for example which I'm not interested in.
If you do a search on Google Flights and pop open the date selector dropdown, you get a calendar showing the best prices for every day for two months, with the cheapest days highlighted in green:
Disclaimer: I work at Google, but not on Flights.