d3.js How to add lines to a bar chart

Question

I have a data set of 4 values. [a,b,c,d]. Currently they are displayed in a bar chart, for each value one bar. Now as values c and d are averages, i would like to display th

Answer 1

I'm posting an example here because none of the answers provide a working example in jsbin or jsfiddle, etc. of a barchart with a line.
http://jsbin.com/gisinomo/1/edit

The example is a fork of the simple Bar Chart on the d3 wiki.
http://bl.ocks.org/mbostock/3885304

CSS

body {
  font: 10px sans-serif;
}

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

.bar {
  fill: steelblue;
}

.x.axis path {
  display: none;
}

.line {
  fill: none;
  stroke: #444;
  stroke-width: 1.5px;
}

JavaScript

var data = [{ "letter": "A", "frequency": .08167},
{ "letter": "B", "frequency": .01492},
{ "letter": "C", "frequency": .02780},
{ "letter": "D", "frequency": .04253},
{ "letter": "E", "frequency": .12702},
{ "letter": "F", "frequency": .02288},
{ "letter": "G", "frequency": .02022},
{ "letter": "H", "frequency": .06094},
{ "letter": "I", "frequency": .06973},
{ "letter": "J", "frequency": .00153},
{ "letter": "K", "frequency": .00747},
{ "letter": "L", "frequency": .04025},
{ "letter": "M", "frequency": .02517},
{ "letter": "N", "frequency": .06749},
{ "letter": "O", "frequency": .07507},
{ "letter": "P", "frequency": .01929},
{ "letter": "Q", "frequency": .00098},
{ "letter": "R", "frequency": .05987},
{ "letter": "S", "frequency": .06333},
{ "letter": "T", "frequency": .09056},
{ "letter": "U", "frequency": .02758},
{ "letter": "V", "frequency": .01037},
{ "letter": "W", "frequency": .02465},
{ "letter": "X", "frequency": .00150},
{ "letter": "Y", "frequency": .01971},
{ "letter": "Z", "frequency": .00074}];

var margin = {top: 20, right: 20, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var formatPercent = d3.format(".0%");

var x = d3.scale.ordinal()
    .rangeRoundBands([0, width], .1);

var x2 = d3.scale.ordinal()
  .rangeBands([0, width], 0);

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

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

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .tickFormat(formatPercent);

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 + ")");


  data.forEach(function(d) {
    d.frequency = +d.frequency;
  });

  x.domain(data.map(function(d) { return d.letter; }));
  x2.domain(data.map(function(d) { return d.letter; }));
  y.domain([0, d3.max(data, function(d) { return d.frequency; })]);

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

  svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Frequency");

  svg.selectAll(".bar")
      .data(data)
    .enter().append("rect")
      .attr("class", "bar")
      .attr("x", function(d) { return x(d.letter); })
      .attr("width", x.rangeBand())
      .attr("y", function(d) { return y(d.frequency); })
      .attr("height", function(d) { return height - y(d.frequency); });

var dataSum = d3.sum(data, function(d) { return d.frequency; }); 

 var line = d3.svg.line()
    .x(function(d, i) { 
      return x2(d.letter) + i; })
    .y(function(d, i) { return y(dataSum/data.length); }); 

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

Answer 2

Make a list of objects with two properties out of your data:

var data = [1,2,3,4];
var objects = data.slice(0, data.length/2).map(
  function(d,i) { return { value:d, average:data[i+data.length/2] }; } 
);

Then you can do something like this (not tested):

var bars = chart.selectAll("g.bar")
   .data(objects)
  .enter().append("g")
    .attr("class", "bar")
    .attr("transform", function(d,i) { return "translate("+(i*10)+",0)"; });


bars.append("rect")
    .attr("x", 0)
    .attr("y", function(d,i) { return height - d.value; })
    .attr("width", 10)
    .attr("height", function(d,i) { return d.value; });

bars.append("line")
    .attr("x1", 0)
    .attr("y1", function(d,i) { return height - d.average; })
    .attr("x2", 10)
    .attr("y2", function(d,i) { return height - d.average; });