step10.js

const lines = document.getElementsByClassName('line');

const mousePerLine = mouseOver
  .selectAll('.mouse-per-line')
	  .data(metrics)
		.enter()
		.append('g')
			.attr('class', 'mouse-per-line');

mousePerLine
  .append('circle')
    .attr('r', 3)
    .style('stroke', d => color(d.name))
    .style('fill', d => color(d.name))
    .style('stroke-width', '1px')
    .style('opacity', '0');

mousePerLine
  .append('text')
    .attr('transform', 'translate(10, 3)');