Nevermind.
sourceforge.net/p/gnuplot/bugs/2634
This is a two-year-old known bug in gnuplot on Ubuntu. Why that specific? Who knows? But the solution is to invoke the gnuplot script in an environment where GDK_BACKEND=x11
That is the solution to allow the gnuplot process's wxt terminal to actually update with every new plot command.
This question is closed.