toOptionsJSON method
Implementation
@override
void toOptionsJSON(StringBuffer buffer) {
  super.toOptionsJSON(buffer);
  if (index != null) {
    buffer.writeAll(['"index":', jsonEncode(index), ','], '');
  }
  if (period != null) {
    buffer.writeAll(['"period":', jsonEncode(period), ','], '');
  }
  if (periods != null) {
    buffer.write('"periods":[');
    for (var item in periods!) {
      buffer.writeAll([jsonEncode(item), ','], '');
    }
    buffer.write('],');
  }
}