getStyle(index, active) {
    const mode = active ? 'active' : 'default';
    return index === undefined && this._cachedMeta.dataset
      ? this.resolveDatasetElementOptions(mode)
      : this.resolveDataElementOptions(index || 0, mode);
  }
Returns a set of predefined style properties that should be used to represent the dataset
or the data if the index is specified
@param {number} index - data index
@param {boolean} [active] - true if hover
@return {object} style object