For each node in each column, check the node's vertical position in relation to its targets and sources vertical position and shift up/down to be closer to the vertical middle of those targets and sources
function relaxLeftAndRight (alpha, id) {
        var columnsLength = columns.length

        columns.forEach(function (nodes) {
          var n = nodes.length
          var depth = nodes[0].depth

          nodes.forEach(function (node) {
            // check the node is not an orphan
            var nodeHeight
            if (node.sourceLinks.length || node.targetLinks.length) {
              if (node.partOfCycle && numberOfNonSelfLinkingCycles(node, id) > 0) {

                /* empty */
              } else if (depth == 0 && n == 1) {
                nodeHeight = node.y1 - node.y0

                node.y0 = y1 / 2 - nodeHeight / 2
                node.y1 = y1 / 2 + nodeHeight / 2
              } else if (depth == columnsLength - 1 && n == 1) {
                nodeHeight = node.y1 - node.y0

                node.y0 = y1 / 2 - nodeHeight / 2
                node.y1 = y1 / 2 + nodeHeight / 2
              } else {
                var avg = 0

                var avgTargetY = mean(
                  node.sourceLinks,
                  linkTargetCenter
                )
                var avgSourceY = mean(
                  node.targetLinks,
                  linkSourceCenter
                )

                if (avgTargetY && avgSourceY) {
                  avg = (avgTargetY + avgSourceY) / 2
                } else {
                  avg = avgTargetY || avgSourceY
                }

                var dy = (avg - nodeCenter(node)) * alpha
                // positive if it node needs to move down
                node.y0 += dy
                node.y1 += dy
              }
            }
          })
        })
      }