creates vertical buffer values per set of top/bottom links
function calcVerticalBuffer (links, circularLinkGap, id) {
    links.sort(sortLinkColumnAscending)
    links.forEach(function (link, i) {
      var buffer = 0

      if (selfLinking(link, id) && onlyCircularLink(link)) {
        link.circularPathData.verticalBuffer = buffer + link.width / 2
      } else {
        var j = 0
        for (j; j < i; j++) {
          if (circularLinksCross(links[i], links[j])) {
            var bufferOverThisLink =
              links[j].circularPathData.verticalBuffer +
              links[j].width / 2 +
              circularLinkGap
            buffer = bufferOverThisLink > buffer ? bufferOverThisLink : buffer
          }
        }

        link.circularPathData.verticalBuffer = buffer + link.width / 2
      }
    })

    return links
  }