getModel method
Gets the selection model. If the model is null, create one from
configuration and the processed seriesList passed in.
Implementation
common.SelectionModel<D> getModel(
    List<common.ImmutableSeries<D>> seriesList) {
  _model ??= common.SelectionModel<D>.fromConfig(
      selectedDataConfig, selectedSeriesConfig, seriesList);
  return _model!;
}