Hi there, you can use the JSON input to transform the values used in that aggregation after the query has been executed, but you can't change the query itself. In other words, it won't allow you to do what you're trying to do. I think the closest you could get is to use Timelion to perform a series difference calculation. This will be in the form of a chart, not a table, but it will show difference values based on different points in time.
I hope this helps,
CJ