Statistic Diagramm doesn't work for some entities?

Hi guys,
I have an entity that shows the watt-hour-consumption for my heat pump.
Looks like this:


I wanted to add an statistic tile to show the Watt consumption together with the heat temperature of my “tap water”. For some very weird reason I cannot combine them. Why?
I can have a statistic for heat temp or for watt consumption. But not both.
The statistic tile doesnt allows me to add both entities. If I show them separetly everything is fine.

Any idea why?