Google Flights does this for you. It can both show you actual prices per calendar month, and predicted price rise/fall. It even highlights the lowest price dates. And you can sign up for e-mailed price alerts.
This is correct. Google Flight is amazing at tracking flight prices over time and lets you see dips in the past on a readable chart.
Something that they could make easier would be to forecast exact price changes based on the same flight flight on other dates.