interpolator in need of interpolation

gistfile1.txt

// use of interpolator:
selection.transition()
	.attrTween('d', areaGenerator)

// interpolator:
areaGenerator = function (d) {
	// do I need a d3.interpolate call here, sim. to https://gist.github.com/mbostock/5100636#file-index-html-L66 ?
	return function (t) {
		return d3.svg.area()
		.x(d => xScale(d.x))
		.y0(innerHeight)
		.y1(d => yScale(d.y))
		.interpolate((points) => {
			console.log(t);	// outputs values from 0 to 1, as expected,
					// but there's no longer any interpolation --
					// the same points are passed into each function call
			
			// symmetrical concave beziers
			let roundness = 0.8,
				steepness = 0.5,
				x0 = points[0][0],
				y0 = points[0][1],
				x1 = points[1][0],
				y1 = points[1][1],
				x2 = points[2][0],
				y2 = points[2][1];
				
			let path = points[0].join(' ') +				// first anchor point
				` C${x0 + steepness*(x1-x0)} ${y0},` +		// first control point, inside curve
				`${x0 + roundness*(x1-x0)} ${y1},` + 		// second control point, outside curve
				points[1].join(' ') +						// middle anchor point
				` C${x1 + (1-roundness)*(x2-x1)} ${y1},` +	// third control point, outside curve
				`${x1 + (1-steepness)*(x2-x1)} ${y2},` +	// fourth control point, inside curve
				points[2].join(' ');						// last anchor point
	
	
			return path;
		})
		(data);
	}
};