orient

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <marker
      id="arrow"
      viewBox="0 0 10 10"
      refX="5"
      refY="5"
      markerWidth="6"
      markerHeight="6"
      orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 z" />
    </marker>

    <marker
      id="dataArrow"
      viewBox="0 0 10 10"
      refX="5"
      refY="5"
      markerWidth="6"
      markerHeight="6"
      orient="-65deg">
      <path d="M 0 0 L 10 5 L 0 10 z" fill="red" />
    </marker>
  </defs>

  <polyline
    points="10,10 10,90 90,90"
    fill="none"
    stroke="black"
    marker-start="url(#arrow)"
    marker-end="url(#arrow)" />

  <polyline
    points="15,80 29,50 43,60 57,30 71,40 85,15"
    fill="none"
    stroke="grey"
    marker-start="url(#dataArrow)"
    marker-mid="url(#dataArrow)"
    marker-end="url(#dataArrow)" />
</svg>