function tweenLoading(b) {
            let i;

            b.innerRadius = 0;
            i = d3Interpolate.interpolate({ startAngle: 0, endAngle: 0}, b);

            return function(t) { return shape(i(t)); };
        }
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