Hello Emiel. A breakdown of charges for resources in your project should be available on your
Billing page. I encourage you to read through the
App Engine Pricing page to understand those charges. Note that you are charged for the resources (instance time) that you use, not for how much utilization they see.
As for the traffic graph you're looking at, the value is an average per second and you can mouse over to any part of the graph to get the average for that time period. It also tends to round down so if your average number of requests per second is below zero, it will show zero.