code: function prepend(tagName) {
    //the same as append, but inserting the element before the first element in the container
    const elements = [];
  
    this.each(function() {
      const document = this.ownerDocument;
      const element = document.createElement(tagName);
      this.insertBefore(element, this.firstChild);
      elements.push(element);
    });
  
    return selectAll(elements);
  }
docs: jQuery equivalent: [$.prepend](http://api.jquery.com/prepend/)
  
  Inserts elements as first child of the current selection. Returns the new elements as a D3 selection.
  
  @function prepend
  @example
  d3.selectAll('li')
  .prepend('a')
  .text('Some Link')
  //do somethin else with the link
  
  @param {string}  tagName - The element to append
  @returns {selection} - The new elements as a D3 selection.