Quiver series — a 2D vector field. Each (x, y) has an arrow
pointing in the direction of (u, v) with length proportional
to √(u² + v²).
Fields:
:xs,:ys— tail positions (length n):us,:vs— vector components at each tail (length n):scale— multiplier applied to every vector before drawing:head_size— arrow-head length in px (default6):stroke,:stroke_width,:label