using attributes ... but I can't figure out how to

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I was happy when I noticed that plotly does allow for entity attributes, which apex doesn't.
So I gave it a try without much luck.

Yes it works with dumb 1-level attributes.
But, while I can use the following in the DevToolstemplate aswell as within template sensors ....

{{ state_attr("sensor.hvl", "HVS")['BMS1']['Umax'] }}
{{ states.sensor.hvl.attributes.HVS.BMS1.Umax }}

I found no way to make this going with plotly's example

type: custom:plotly-graph
entities:
  - entity: sensor.hvl
    attribute: ?????

Any idea about what to replace the ?????? with? Tried a lot but found no solution to make structured attributes working.