Thank you for the hint. Lookup in the Maxima docs confirms that unboxed plots ignore axis labels.
After messing around with smart placement of [label, ["text", x,y]] I gave up and ended up with completely abandoning annotations in the graph, because iterative adjustment for random plots isn't something which I want to go through every time when I make such a plot.
Instead I put a text line with explanations above the plot. This gives me a level of control which plot() never will.