code: function after(tagName) {
    const elements = [];
  
    this.each(function() {
      const document = this.ownerDocument;
      const element = document.createElement(tagName);
      this.parentNode.insertBefore(element, this.nextSibling);
      elements.push(element);
    });
  
    return selectAll(elements);
  }
docs: jQuery equivalent: [$.after](http://api.jquery.com/after/)
  
  Inserts new elements after each element in the current selection. Returns the newly created elements as a d3 selection.
  
  @function after
  @example
  d3.selectAll('li')
  .after('li')
  .text('Item');
  //do something else with the inserted elements...
  
  @param {string}  tagName - The element to insert
  @returns {selection} - The new elements as a D3 selection.