clip-path: polygon() can fill a shape. It cannot stroke one. So pure-CSS chart lines in FSCSS (and libraries like st-core) fake a stroke with a dual-pass polygon : walk the data forward for the top edge, walk it backward for a parallel edge offset by line width, fill the thin band between them. This post unpacks that pattern with @arr , inline() , $ , count() , and .reverse — the same building blocks used in st-core’s @st-chart-line . The problem You have poin...