Thanks Pete.

I'm feeling kind of embarrassed as I've obviously misinterpreted what it was intended to do. I'd assumed that "displayed samples" denoted the first and last samples in the currently displayed timeline graph window, so was expecting it to change as I scrolled or zoomed that pane. I now realise that it relates to the samples used as input to the upper (trace) graph. Apologies for the wild goose chase.

Gil