function appendArrow(d, i) {
  let arrowHeadSize = strokeWidth(d.value) * 3;

  let thisPath = d3.select(this).node();
  let pathLength = thisPath.getTotalLength();
  let point = thisPath.getPointAtLength(pathLength);
  let previousPoint = thisPath.getPointAtLength(pathLength - 1);

  let rotation = 0;

  if (point.y == previousPoint.y) {
    rotation = point.x < previousPoint.x ? 180 : 0;
  } else if (point.x == previousPoint.x) {
    rotation = point.y < previousPoint.y ? -90 : 90;
  } else {
    let adj = Math.abs(point.x - previousPoint.x);
    let opp = Math.abs(point.y - previousPoint.y);
    let angle = Math.atan(opp / adj) * (180 / Math.PI);

    if (point.x < previousPoint.x) {
      angle = angle + (90 - angle) * 2;
    }
    if (point.y < previousPoint.y) {
      rotation = -angle;
    } else {
      rotation = angle;
    }
  }

  let dString =
    "M" +
    point.x +
    "," +
    (point.y - arrowHeadSize / 2) +
    " L" +
    (point.x + arrowHeadSize / 2) +
    "," +
    point.y +
    " L" +
    point.x +
    "," +
    (point.y + arrowHeadSize / 2);

  let rotationString =
    "rotate(" + rotation + "," + point.x + "," + point.y + ")";

  g.append("path")
    .attr("d", dString)
    .attr("class", "arrow-head")
    .attr("transform", rotationString)
    .style("stroke", colour(d.source))
    .style("fill", colour(d.source));
}