It would be cool, if we could set the signal which should be displayed via circiuts :D

This is something I always wanted too from the original mod and it was something I wanted to look into now I kinda unofficially took over.

How do you think it should work? Only thing I thought of is whatever signal has the highest value is what gets shown.

Choosing the signal with the highest value sounds good.

Alrighty, no promise this gets done in a reasonable timeframe. I have no idea how the circuit stuff works, but I'll be working on it when I have free time for sure.

Why not take a look at display panel in vanilla game? It will choose the signal in a specific order, though not sorting from high to low, but at least the plates could display by circuit?

Yeah that was my next plan, honestly I genuinely suck at Lua so that's just my main issue.

I can maintain the mod functionally but changing it is a bit harder for me.

It's still in process tho, I've not given up despite how embarrassingly long this is taking me. Sorry for the long wait :(

Update on this. Should be available by the end of the week, Sunday by my definition, at the latest.

This does come with some rather drastic changes. Mainly the UI is no longer custom, it will just be what is provided by Factorio. Which with the way its being implemented, is exactly the same as the vanilla display panel that was mentioned.
This still lets you set an icon, still lets you have a world marker, though I can't imagine very many people used that, I know I never did.

But now we have the added bonus of at least being able to control it via circuits. What you can do with the display panel in vanilla is what you can do with the display plates.

Edit: I should warn, this is a huge breaking change. Plates will need to be updated in game manually, as in their signals. The plates themselves should in theory be there still. Sorry for that inconvenience!

1.6.0 released, unfortunately with the current API i don't think it's possible to migrate the old states.
The old implementation was rather, hacked together I'd say.
No easy way to obtain the plate's icon information through migrations or the configuration event.

This does mean you will have to redo all your plates again, of which you will get a message when you load a save with this update. I am sorry for this.