Wrong value in graph with bars

Hi

I have use the difference function because it was so described here.

Now i have try it with derivate but it shows me wrong values.

What type of aggregation I have to choose?
I do not want the average i need the exactly value from each day.
Mean is the wrong aggregation I know that now.
What is the right setting to display the daily consumption correctly?

Thanks for the clarification because of the time zone.