docs: function nestRollupAggregateSum(array) {
  		// Note: we don't use d3.sum here because we want to end up with a null value for "holes"
  		var sum = null, plus = null, minus = null, 
  			d, v, i, len = array.length, groupId, scale, negate = false;
  		for ( i = 0; i < len; i += 1 ) {
  			d = array[i];
  			v = d[parent.plotPropertyName];
  			if ( v !== undefined ) {
  				groupId = d[parent.internalPropName].groupId;
  				scale = parent.scaleFactor(groupId);
  				negate = negativeGroupMap[groupId] === true;
  				if ( negate ) {
  					minus += v * scale;
  				} else {
  					plus += v * scale;
  				}
  			}
  		}
  		if ( plus !== null || minus !== null ) {
  			sum = plus - minus;
  		}
  		return { date : array[0].date, y : sum, plus : plus, minus : minus };
  	}