Allow for 'Virtual' Entities

[Madelena]

Creating a detailed Sankey Chart requires making template sensors with the sum of the values of its children. It would be nice to not having such requirement, as I end up having to create a ton of templates that are only used for this chart but nowhere else.

Describe the solution you'd like
It will be nice to have the ability to point to a node without an entity_id:

sections:
  - entities:
    - entity_id: sensor.house_total
      children:
        - node.floor1
        - node.floor2
        - node.outdoors
  - entities:
    - node: floor1
      children:
        - sensor.airconditioner
        - sensor.stove
    - node: floor2
      children:
        - sensor.computer
        - sensor.bedlight
    - node: outdoors
      children:
        - sensor.floodlights
  - entities:
    - sensor.airconditioner
    - sensor.stove
    - sensor.computer
    - sensor.bedlight
    - sensor.floodlights

Describe alternatives you've considered
I created a dozen template entities that are sum of the values of various sensors.

[MindFreeze]

I can see how this would make setup easier for some cases but it seems like a lot of effort for very little gain.
I will leave this issue open in case someone wants to submit a PR or I inexplicably find myself with a lot of free time but I consider it very low priority.

[emufan]

Great Card!

This improvement here would be more than great. And should include perhaps "other/rest" as well, As I have a total, and some single values, but there is a big rest and I don't have an entity for this, but would like to see it as well,

[MindFreeze]

For other/rest see #2 . I plan implementing it because it is a bit cumbersome to do with templates. Virtual grouping (this issue) however is low priority because a template for it in HA is just a simple sum of sensors and many people would already have it.

[tadasdanielius]

After integration with the energy dashboard this feature would be very useful. Currently, you need to add the "sum template" to the energy dashboard individual device list in order to be able to use it in the diagram. Adding those sums to the individual device list will show the energy usage twice.

[MindFreeze]

I recently laid the groundwork for this and it may even be possible already but haven't tested it. In theory if a bunch of entities have a single parent with type: remaining_child_state, this should create a virtual group. But not tested yet.

[MindFreeze]

This grouping works in both directions now. Added a FAQ section to README with some explanation