Show another entity as secondary info

[fusionstream]

Checklist

• I am running the latest version of the card
• I checked if there isn't another feature request opened with the same request
• I checked that the feature is not already available in the dev branch

Is your feature request related to a problem? Please describe.
I am drawing a graph of WiFi strength of various devices. I can see that some devices have a weak signal but I am unable to see what AP they are connected to.

Describe the solution you'd like
Have the ability to show another entity (or an attribute) as secondary info in the legend

Describe alternatives you've considered
None.

Additional context
None.