A black cubic Bézier path is drawn on an SVG canvas using D3 v4, with its total length queried via `getTotalLength()`. The example marks two points sampled along the path at 50% and 78% of its length, using `getPointAtLength()` to position small red and blue circles. A green line is then constructed between the two points and extended backwards to the canvas edge, demonstrating how to compute a secant line from arbitrary positions along a vector path. The visualization uses inline SVG elements created through D3's selection API, with styling applied via a linked CSS file.