New to ES/Compare daily data to previous date's data

Okay, it looks like abdon's suggestion to use the Derivative function was successful at showing the difference from one moment to the next (thank you again!). Now my question is if there's a way I can open up that function to more complex calculations. So whereas the function for derivative is a basic t(n+1) - t(n) (I THINK I wrote that right...), is there a way to access the function so that I could calculate other things (like a percentage change, for example)? In other words, right now Derivative aggregations are a fixed operation. I want to find some kind of editable function. Does that exist?