code: function tweenLoading(b) {
              let i;
  
              b.innerRadius = 0;
              i = d3Interpolate.interpolate({ startAngle: 0, endAngle: 0}, b);
  
              return function(t) { return shape(i(t)); };
          }
docs: Animation for chart loading
  Check out {@link http://bl.ocks.org/mbostock/4341574| this example}
  
  @param  {Object} b Data point
  @return {Function}   Tween function
  @private