Sorry, I have both 1.7.2 and 1.8.0-beta3 installed, and I switched between my messages. The problem occurs with both, and the solution fixes the problem for both. The code using GR directly without Plots has the same problem, and is fixed by copying the library. This is not a GR or Plot issue, I believe. I think (?) it has to do with libraries having been updated on my rolling release Linux (Debian testing). Thanks for GR.jl, love it!
I found the solution here: GLIBCXX_3.4.26 not found in RCall - #10 by laborg