如何在鼠标悬停时获取直方图中数据元素的索引？

var color = "steelblue"; 
var f = d3.random.normal(20, 5);
// Generate a 1000 data points using normal distribution with mean=20, deviation=5
var values = d3.range(1000).map(id => ({
                                id: id,
                                value: f()
}));
var valFn = d => d.value;

// A formatter for counts.
var formatCount = d3.format(",.0f");

var margin = {top: 20, right: 30, bottom: 30, left: 30},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var max = d3.max(values, valFn);
var min = d3.min(values, valFn);
var x = d3.scale.linear()
      .domain([min, max])
      .range([0, width]); 

// Generate a histogram using twenty uniformly-spaced bins.
var data = d3.layout.histogram()
    .bins(x.ticks(20))
    .value(valFn)
    (values);

var yMax = d3.max(data, function(d){return d.length});
var yMin = d3.min(data, function(d){return d.length});
var colorScale = d3.scale.linear()
            .domain([yMin, yMax])
            .range([d3.rgb(color).brighter(), d3.rgb(color).darker()]);

var y = d3.scale.linear()
    .domain([0, yMax])
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var svg = 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 + ")");

var bar = svg.selectAll(".bar")
    .data(data)
  .enter().append("g")
    .attr("class", "bar")
    .attr("transform", function(d) { return "translate(" + x(d.x) + "," + y(d.y) + ")"; })
    .on("mouseover", d => { console.log(d)});

bar.append("rect")
    .attr("x", 1)
    .attr("width", (x(data[0].dx) - x(0)) - 1)
    .attr("height", function(d) { return height - y(d.y); })
    .attr("fill", function(d) { return colorScale(d.y) });

bar.append("text")
    .attr("dy", ".75em")
    .attr("y", -12)
    .attr("x", (x(data[0].dx) - x(0)) / 2)
    .attr("text-anchor", "middle")
    .text(function(d) { return formatCount(d.y); });

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

/*
* Adding refresh method to reload new data
*/
function refresh(values){
  // var values = d3.range(1000).map(d3.random.normal(20, 5));
  var data = d3.layout.histogram()
    .value(valFn)
    .bins(x.ticks(20))
    (values);

  // Reset y domain using new data
  var yMax = d3.max(data, function(d){return d.length});
  var yMin = d3.min(data, function(d){return d.length});
  y.domain([0, yMax]);
  var colorScale = d3.scale.linear()
              .domain([yMin, yMax])
              .range([d3.rgb(color).brighter(), d3.rgb(color).darker()]);

  var bar = svg.selectAll(".bar").data(data);

  // Remove object with data
  bar.exit().remove();

  bar.transition()
    .duration(1000)
    .attr("transform", function(d) { return "translate(" + x(d.x) + "," + y(d.y) + ")"; });

  bar.select("rect")
      .transition()
      .duration(1000)
      .attr("height", function(d) { return height - y(d.y); })
      .attr("fill", function(d) { return colorScale(d.y) });

  bar.select("text")
      .transition()
      .duration(1000)
      .text(function(d) { return formatCount(d.y); });

}

// Calling refresh repeatedly.
setInterval(function() {
  var values = d3.range(1000).map(id => ({
                                  id: id,
                                  value: f()
  }));
  refresh(values);
}, 2000);

body {
  font: 10px sans-serif;
}

.bar rect {
  shape-rendering: crispEdges;
}

.bar text {
  fill: #999999;
}

.axis path, .axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.as-console-wrapper {
  height: 20%;
}

<script src="https://d3js.org/d3.v3.min.js"></script>