A line animates from source to target as a blue stroke draws itself, while a red arrowhead travels along the same path. The arrow rotates to match the line's direction using `transition.attrTween` and `translateAlong`, which queries the path's geometry with `getPointAtLength`. The line's progress is shown via a `stroke-dasharray` offset transition. The example uses d3.v4's SVG and animation APIs, with the arrow marker defined as a custom `svg:marker` as well as a separate path.