html`<svg width=640 height=120 stroke=black>
<marker id=arrow markerHeight=10 markerWidth=10 refX=9 refY=3 orient=auto>
<path d="M0,0L9,3L0,6Z" />
</marker>
${d3.pairs(
Array.from(
{length: 20},
(_, i) => [i / 10 * 640, Math.random() * 120]
),
([x1, y1], [x2, y2]) => svg`<line
marker-end=url(${location}#arrow)
x1=${x1} y1=${y1}
x2=${x2} y2=${y2}>`
)}
</svg>`