Thanks for the info. I confirmed that az.plot_trace() works for my other traces, but the difficult one I have still does not plot, even if I set compact=True. I almost froze my whole OS just now testing that out, and I had to forcibly shut down my Python kernel once I noticed the session froze.
Pymc3 traces have literally never given me such trouble before, for just one variable (no matter how multi-dimensional they were).