code: function tweenArc(a) {
              let i = d3Interpolate.interpolate(this._current, a);
  
              this._current = i(0);
  
              return function(t) {
                  return shape(i(t));
              };
          }
docs: Stores current angles and interpolates with new angles
  Check out {@link http://bl.ocks.org/mbostock/1346410| this example}
  
  @param  {Object}     a   New data for slice
  @return {Function}       Tweening function for the donut shape
  @private