calculated formula problem

[atreyu]

Hi guys,

I have to calculate the difference between two values of the same item, the last value - the last value before the last, is that possible?

To be more clear;

12:33 - sensor[VBAT] = 12.1V
12:38 - sensor[VBAT] = 12.3V

I want to calculate, then graph, the difference between sensor[VBAT] @12:38 and sensor[VBAT] @12:33.

my formula looks something like: last("sensor[VBAT],#1")-last("sensor[VBAT],#2"), but it doesn't seem to work and I can't find any documentation related to this, but the three examples in the official documentation which do not relate to my needs. The formula used I found on a forum, but I have my doubts about it since it returns "not supported"

Any ideas would be much appreciated.

Thanks