Thanks, Hans Milton. That's the solution, alright. Please expand on the way you entered the Symbolize[]. I was already using the keyboard shortcut ctrl+& and _. That wasn't the problem. But there was something else you did that created the box in which the x-bar was entered. That now appears to have been what I missed from reading the documentation.

That's it. Using the notation pallet works. Typing it doesn't. There's a ParsedBoxWrapper[] function revealed in raw input form,...

Symbolize[ParsedBoxWrapper[OverscriptBox["x", "_"]]]

Documentation for ParsedBoxWrapper[] is comically circular. But it has a Tech Note link to a tutorial that explains everything:

Complex Patterns and Advanced Features

I'm not sure, but I think it is not possible to create a variable with a typeset name like h₂o in Wolfram language. If someone knows how, please reply.

Thanks, Gianluca Gorni. I should clarify. The goal wasn't just to render the plot. I'm trying to become proficient with Wolfram's typeset notation, that is so much more pleasing to the mathematician's eyes. I think Hans Milton has the solution once it's explained more fully.