code: function() {
  	var _findYear = function(years) {
  		return _.toInt( _.reduce(years, function(res, year) {
  			return res || ((this._years[year].sum() > 0) ? year : undefined);
  		}, undefined, this) );
  	};
  	
  	if (!_findYear.call(this, _.keys(this._years))) {
  		return [];
  	}
  	return _.range( _findYear.call(this, _.keys(this._years)), (_findYear.call(this, _.keys(this._years).reverse() )+1) );
  }
docs: return the beginning of the range at the first accumulated value and the end of the range at the last accumulated value;