_getRotationExtents() {
    let min = TAU;
    let max = -TAU;

    for (let i = 0; i < this.chart.data.datasets.length; ++i) {
      if (this.chart.isDatasetVisible(i) && this.chart.getDatasetMeta(i).type === this._type) {
        const controller = this.chart.getDatasetMeta(i).controller;
        const rotation = controller._getRotation();
        const circumference = controller._getCircumference();

        min = Math.min(min, rotation);
        max = Math.max(max, rotation + circumference);
      }
    }

    return {
      rotation: min,
      circumference: max - min,
    };
  }
Get the maximal rotation & circumference extents
across all visible datasets.