code: function(el, options) {
      var self = this;
      if (typeof options === 'undefined' ) {
          options = {};
      }
  
      this.xfield = options.xfield || 'time';
      this.yfield = options.yfield || 'value';
      this.width = options.width || 1;
      this.type = 'slide';
      this.duration = options.duration || 0;
  
      this.el = el;
  
      this.selection = d3.select(el);
  
      this.line = d3.svg.line();
  
      this.line.defined(function(d) {
          return d[self.yfield] !== INTERPOLATION_BREAK;
      });
  
      this.series = this.selection.append('g')
          .attr('class', 'series');
  
      this._attributes = options;
  
      if (options.label) {
          this.series.attr('id', options.label);
      }
  
      this.data = [];
      this.draw_range = undefined;
  
      this.hover_point = null;
  }
docs: line constructor