code: function(el, options) {
      var defaults = require('../utils/default-options')();
  
      if (typeof options === 'undefined') {
          options = {};
      }
  
      this.options = _.extend(defaults, options);
      this.color = this.options.color;
  
      this.xAccessor = this.options.xAccessor;
      this.yAccessor = this.options.yAccessor;
  
      this.el = el;
  
      this.selection = d3.select(el);
  
      this.transform = 0;
  
      this._data = [];
  
      this.area = d3.svg.area();
  
      this.zeroArea = d3.svg.area();
  
      this.series = this.selection.append('g')
          .attr('class', 'series')
          .attr('clip-path', 'url(#areaClip)');
  }
docs: area constructor -- pretty much the same as line (area is just a line path with closing ends.)