segmentPointIndex method
Implementation
@override
int segmentPointIndex(Offset position, ChartSegment segment) {
  final SplineRangeAreaSegment splineRangeAreaSegment =
      segment as SplineRangeAreaSegment;
  final int index = _computePointIndex(
    splineRangeAreaSegment._lowPoints,
    position,
  );
  if (index != -1) {
    return index;
  }
  return _computePointIndex(splineRangeAreaSegment._highPoints, position);
}