dist/d3-sankey-circular.es.js
function linkPerpendicularYToLinkTarget(longerLink, shorterLink) {
  // get the angle for the longer link
  var angle = linkAngle(longerLink);

  // get the adjacent length to the other link's x position
  var heightFromY1ToPependicular = linkXLength(shorterLink) / Math.tan(angle);

  // add or subtract from longer link's original y1, depending on the slope
  var yPerpendicular = incline(longerLink) == 'up' ? longerLink.y1 - heightFromY1ToPependicular : longerLink.y1 + heightFromY1ToPependicular;

  return yPerpendicular;
}
function linkPerpendicularYToLinkTarget(longerLink, shorterLink) {
  // get the angle for the longer link
  var angle = linkAngle(longerLink);

  // get the adjacent length to the other link's x position
  var heightFromY1ToPependicular = linkXLength(shorterLink) / Math.tan(angle);

  // add or subtract from longer link's original y1, depending on the slope
  var yPerpendicular = incline(longerLink) == 'up' ? longerLink.y1 - heightFromY1ToPependicular : longerLink.y1 + heightFromY1ToPependicular;

  return yPerpendicular;
}
Return the Y coordinate on the longerLink path * which is perpendicular shorterLink's source. * approx, based on a straight line from target to source, when in fact the path is a bezier