Edit in JSFiddle

var data = [
  {"y":0.5074666906113496,"x":"2017-03-20T23:00:00.000Z"},
  {"y":0.9527321029510436,"x":"2017-03-21T00:00:00.000Z"},
  {"y":0.7600950959703447,"x":"2017-03-21T01:00:00.000Z"},
  {"y":0.20580227686108898,"x":"2017-03-21T02:00:00.000Z"},
  {"y":0.5733164708745364,"x":"2017-03-21T03:00:00.000Z"},
  {"y":0.7564259320114604,"x":"2017-03-21T04:00:00.000Z"},
  {"y":0.6267454112244419,"x":"2017-03-21T05:00:00.000Z"},
  {"y":0.11944023054210295,"x":"2017-03-21T06:00:00.000Z"},
  {"y":0.9297418072540851,"x":"2017-03-21T07:00:00.000Z"},
  {"y":0.8884353531688012,"x":"2017-03-21T08:00:00.000Z"},
  {"y":0.5619578787536637,"x":"2017-03-21T09:00:00.000Z"},
  {"y":0.23583955393871814,"x":"2017-03-21T10:00:00.000Z"},
  {"y":0.2680568515809374,"x":"2017-03-21T11:00:00.000Z"},
  {"y":0.9129909442537631,"x":"2017-03-21T12:00:00.000Z"},
  {"y":0.80968968597973014,"x":"2017-03-21T13:00:00.000Z"},
  {"y":0.6812223330198619,"x":"2017-03-21T14:00:00.000Z"},
  {"y":0.8875838775047455,"x":"2017-03-21T15:00:00.000Z"},
  {"y":0.5664612526995818,"x":"2017-03-21T16:00:00.000Z"},
  {"y":0.4302245708963415,"x":"2017-03-21T17:00:00.000Z"},
  {"y":0.942675240989681,"x":"2017-03-21T18:00:00.000Z"}
];

var	margin = {top: 20, right: 20, bottom: 30, left: 50};
var width = 550 - margin.left - margin.right;
var height = 350 - margin.top - margin.bottom;
var	x = d3.time.scale().range([0, width]);
var	y = d3.scale.linear().range([height, 0]);
var	svg = addSvg();

addDomains();
addValueLine();
addAxes();
addMouseAction();

function addSvg() {
	return d3.select("body")
    .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 + ")");
}

function addValueLine() {
	var valueline = d3.svg.line()
    .x(function(d) { return x(new Date(d.x)); })
    .y(function(d) { return y(d.y); });

  svg.append("path")	
    .attr("class", "line")
    .attr("d", valueline(data));
}

function addDomains() {
  x.domain(d3.extent(data, function(d) { return new Date(d.x); }));
  y.domain([0, d3.max(data, function(d) { return d.y; })]);
}

function addAxes() {
  var	xAxis = d3.svg.axis().scale(x).orient("bottom").ticks(5);
  var	yAxis = d3.svg.axis().scale(y).orient("left").ticks(5);

  svg.append("g")		
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

  svg.append("g")		
    .attr("class", "y axis")
    .call(yAxis);
}

function addMouseAction() {
	var focusLine = addFocusLine();
  var point = addPoint();
  var bisectDate = d3.bisector(function (d) {
    return new Date(d.x);
  }).left

  svg.append('rect')
    .attr('class', 'overlay')
    .attr('transform', 'translate(' + (-15) + ',0)')
    .attr('width', width + 30)
    .attr('height', height)
    .attr('style', 'fill: none; pointer-events: all')
    .on('mouseout', function () {
    	point.style('display', 'none');
    	focusLine.style('display', 'none');
    })
    .on('mousemove', function () {
      var mouse = d3.mouse(this);
      var mouseX = mouse[0];
      var mouseY = mouse[1];
      var mousePos = mouseX - 15;
      var mouseAsDates = [x.invert(mousePos), x.invert(mouseY)];
      var index = bisectDate(data, mouseAsDates[0], 0);
      var d0 = data[index - 1];
      var d1 = data[index];
      
      if (!d0 || !d1) {
      	return;
      }

			point.style('display', 'block');
      focusLine.style('display', 'block');
    	focusLine.attr('transform', 'translate(' + mousePos + ',' + 0 + ')');
      
      var firstPoint = {w: x(new Date(d0.x)), h: y(d0.y)};
    	var secondPoint = {w: x(new Date(d1.x)), h: y(d1.y)};
      var yPos = getPositionByThales(mousePos, firstPoint, secondPoint);
      point.attr('transform', 'translate(' + mousePos + ',' + yPos + ')');
  	});
}

function addFocusLine() {
  return svg.append('line')
    .attr('y0', 0)
    .attr('y1', height)
    .attr('x0', 0)
    .attr('x1', 0)
    .style('stroke-width', 1)
    .style('stroke', 'rgba(0,0,0,0.6)')
    .style('fill', 'none');
}

function addPoint() {
	return svg.append('g')
    .append('circle')
    .style('display', 'none')
    .attr('r', 4)
    .attr('fill', 'red');
}

function getPositionByThales(currentWidth, previousPoint, currentPoint) {
	var firstHeight = previousPoint.h - currentPoint.h;
  var widthBetweenCursorAndNextPoint = currentPoint.w - currentWidth;
  var widthBetweenPoints = currentPoint.w - previousPoint.w;
  var secondHeight = (firstHeight * widthBetweenCursorAndNextPoint) / widthBetweenPoints;

  return secondHeight + currentPoint.h;
}

body { font: 12px Verdana;}
path { 
  stroke: steelblue;
  stroke-width: 2;
  fill: none;
}
.axis path, .axis line {
	fill: none;
	stroke: grey;
	stroke-width: 1;
}