How to find average acceleration or function that describes the behavior of the acceleration?

I'm doing a study where I need to calculate the HIC (Head Injury Criterion) and for that I need to calculate the average acceleration between an interval of 15 or 36 milliseconds. I did a simulation of a Sled Test and got data for acceleration, but I just got points and formula HIC the acceleration average is calculated through a an integral which integrates the function of acceleration a(t).

Is there any way to find a function that describes the behavior of my acceleration or how to calculate the average acceleration between a time using the Hypergraph or any other method?

Thank you.