defaultRingSizes () {
        return {
            partitionDy: () => this._radius * this._radius,
            scaleInnerRadius: d => d.data.path && d.data.path.length === 1 ?
                this._innerRadius :
                Math.sqrt(d.y0),
            scaleOuterRadius: d => Math.sqrt(d.y1),
            relativeRingSizesFunction: () => []
        };
    }
Constructs the default RingSizes parameter for {@link SunburstChart#ringSizes ringSizes()},
which makes the rings narrower as they get farther away from the center.

Can be used as a parameter to ringSizes() to reset the default behavior, or modified for custom ring sizes.

@example
  var chart = new dc.SunburstChart(...);
  chart.ringSizes(chart.defaultRingSizes())
@returns {RingSizes}