prefer "Isabelle DejaVu Sans", even for headless batch-build (session_graph.pdf);

clarified iterations: enforce full top_down/bottom_up cycle for better stability of layout;

always swap panels, which leads to slightly better GUI layout;

option graphview_swap_panels;

more explicit Layout.Info: size and content;
allow multi-line vertex label, based on content;
misc tuning;

configurable options;

GUI.imitate_font: more explicit result size, e.g. relevant for caching;
some graphview font options: Helvetica family is important for self-contained PDF;
tuned;