function _buildStackLine(source) {
  const {scale, index, line} = source;
  const points = [];
  const segments = line.segments;
  const sourcePoints = line.points;
  const linesBelow = getLinesBelow(scale, index);
  linesBelow.push(_createBoundaryLine({x: null, y: scale.bottom}, line));

  for (let i = 0; i < segments.length; i++) {
    const segment = segments[i];
    for (let j = segment.start; j <= segment.end; j++) {
      addPointsBelow(points, sourcePoints[j], linesBelow);
    }
  }
  return new LineElement({points, options: {}});
}
@param {{ chart: Chart; scale: Scale; index: number; line: LineElement; }} source
@return {LineElement}