This is not a valid edge case.
Ranked Choice has no supported graph in core currently - by choice.
It is never meant be to be visualised with a pie chart. Ranked Choice consists of rounds and that can’t be represented on a single pie chart.
The bug here, if any, is likely someone is able to select a graph when the interface should have prevented them. Has someone changed the setup code to permit that? That said you were always able to “hack” the initial markdown to change attributes post “wizard” (not good)
I did offer an appropriate graph, a Sankey a while back which was working but Discourse declined to merge it because it adds a (albeit well supported and popular) dependent library. Fair enough.
Happy for someone to fund me to bring that back to modern code and prepare a PR if CDCK were to change its position on incorporating the Sankey.
My fork of the plugin shows a Sankey for Ranked Choice. It is likely very out of date and in need of maintenance as no-one is funding its maintenance:
Here are a representative selection of changes for Sankey:
Feel free to do with that what you will.
It is pretty cool if you don’t mind me saying so myself.
It’s definitely better than a pie chart!! 
I recommend this be moved to Feature with the feature request being:
- add an appropriate visualisation for Ranked Choice.