line dual hover

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v5.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

// set the dimensions and margins of the graph
var margin = {top: 10, right: 30, bottom: 30, left: 60},
    width = 460 - margin.left - margin.right,
    height = 400 - margin.top - margin.bottom;

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform",
          "translate(" + margin.left + "," + margin.top + ")");

//Read the data
d3.csv("https://raw.githubusercontent.com/jwilber/random_data/master/tweets.csv").then((data) => {

// Add X scale
  var xs = d3.scaleLinear()
    .domain([1,10])
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(xs));

// Add Y scale
  var ys = d3.scaleLinear()
    .domain([0, 30])
    .range([ height, 0 ]);
  svg.append("g") 
    .call(d3.axisLeft(ys));
  
  // define line generator
  var line = d3.line()
    .x(function(d, i) { 
      return xs(d.day); 
    }) // set the x values for the line generator
    .y(function(d) { return ys(d.tweets); }) // set the y values for the line generator 
    .curve(d3.curveMonotoneX)

var line2 = d3.line()
    .x(function(d, i) { 
      return xs(d.day); 
    }) // set the x values for the line generator
    .y(function(d) { return ys(d.tweets * 2); }) // set the y values for the line generator 
    .curve(d3.curveMonotoneX)
 
  svg
    .append("path")
    .datum(data)
  	.attr('fill', 'none')
    .attr("stroke", "coral")
    .attr("stroke-width", 4.5)
    .attr("d", line)
  
  svg
    .append("path")
    .datum(data)
  	.attr('fill', 'none')
    .attr("stroke", "coral")
    .attr("stroke-width", 4.5)
    .attr("d", line2)
  
  
  
//   This allows to find the closest X index of the mouse:
  var bisect = d3.bisector(d => d.day).left;

//   // Create the circle that travels along the curve of chart
  var focus = svg
    .append('g')
    .append('circle')
      .style("fill", "skyblue")
      .attr("stroke", "black")
      .attr('r', 8.5)
      .style("opacity", 0)
  var focus2 = svg
    .append('g')
    .append('circle')
      .style("fill", "skyblue")
      .attr("stroke", "black")
      .attr('r', 8.5)
      .style("opacity", 0)

//   // Create the text that travels along the curve of chart
  var focusText = svg
    .append('g')
    .append('text') 
      .style("opacity", 0)
      .attr("text-anchor", "left")
      .attr("alignment-baseline", "middle")

//   // Create a rect on top of the svg area: this rectangle recovers mouse position
  svg
    .append('rect')
    .style("fill", "none")
    .style("pointer-events", "all")
    .attr('width', width)
    .attr('height', height)
    .on('mouseover', mouseover)
    .on('mousemove', mousemove)
    .on('mouseout', mouseout);

// What happens when the mouse move -> show the annotations at the right positions.
  function mouseover() {
    focus.style("opacity", 1)
    focus2.style('opacity', 1)
    focusText.style("opacity",1)
  }

function mousemove() {
    // recover coordinate we need
    var x0 = xs.invert(d3.mouse(this)[0]);
    var i = bisect(data, x0, 1);
    selectedData = data[i]
    focus
      .attr("cx", xs(selectedData.day))
      .attr("cy", ys(selectedData.tweets))
    focus2
      .attr("cx", xs(selectedData.day))
      .attr("cy", ys(2* selectedData.tweets))
    focusText
      .html("x:" + selectedData.day + "  -  " + "y:" + selectedData.tweets)
      .attr("x", xs(selectedData.day)+15)
      .attr("y", ys(selectedData.tweets))
    }
  function mouseout() {
    focus.style("opacity", 0)
    focusText.style("opacity", 0)
  }

})
  </script>
</body>

https://d3js.org/d3.v5.min.js