code: function resolveCollisions () {
          columns.forEach(function (nodes) {
            var node, dy, y = y0, n = nodes.length, i
  
            // Push any overlapping nodes down.
            nodes.sort(ascendingBreadth)
  
            for (i = 0; i < n; ++i) {
              node = nodes[i]
              dy = y - node.y0
  
              if (dy > 0) {
                node.y0 += dy
                node.y1 += dy
              }
              y = node.y1 + py
            }
  
            // If the bottommost node goes outside the bounds, push it back up.
            dy = y - py - y1
            if (dy > 0) {
              (y = node.y0 -= dy), (node.y1 -= dy)
  
              // Push any overlapping nodes back up.
              for (i = n - 2; i >= 0; --i) {
                node = nodes[i]
                dy = node.y1 + py - y
                if (dy > 0) (node.y0 -= dy), (node.y1 -= dy)
                y = node.y0
              }
            }
          })
        }
docs: For each column, check if nodes are overlapping, and if so, shift up/down