How do i create a custom graph using others metrics as source?

today i have this AWS cost graph and i wanna make a graph with a initial wallet of 300$ and see this graph go down when this AWS cost graph goes up

literally a i want a graph that starts at 300 and goes down with the current total cost

In theory:
you will use Cloudwatch metric math - Grafana implementation (available from Grafana 5.3) with expression:

300 - metric


Thank you very much

I will study grafana a little bit more before performing the upgrade