d3-example

barchart.css

.bar {
  fill: steelblue;
}

.bar:hover {
  fill: brown;
}

.axis {
  font: 10px sans-serif;
}

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

.x.axis path {
  display: none;
}

.d3-tip {
  line-height: 1;
  font-weight: bold;
  padding: 12px;
  background: rgba(0, 0, 0, 0.8);
  color: #fff;
  border-radius: 2px;
}

/* Creates a small triangle extender for the tooltip */
.d3-tip:after {
  box-sizing: border-box;
  display: inline;
  font-size: 10px;
  width: 100%;
  line-height: 1;
  color: rgba(0, 0, 0, 0.8);
  content: "\25BC";
  position: absolute;
  text-align: center;
}

/* Style northward tooltips differently */
.d3-tip.n:after {
  margin: -1px 0 0 0;
  top: 100%;
  left: 0;
}
pre{
border: 1px solid red; border-radius: 0.5em;
	width:50%;
	padding:20px;
	margin-left:30px;
}

d3.tip.v0.6.3.js

// d3.tip
// Copyright (c) 2013 Justin Palmer
//
// Tooltips for d3.js SVG visualizations

// Public - contructs a new tooltip
//
// Returns a tip
d3.tip = function() {
  var direction = d3_tip_direction,
      offset    = d3_tip_offset,
      html      = d3_tip_html,
      node      = initNode(),
      svg       = null,
      point     = null,
      target    = null

  function tip(vis) {
    svg = getSVGNode(vis)
    point = svg.createSVGPoint()
    document.body.appendChild(node)
  }

  // Public - show the tooltip on the screen
  //
  // Returns a tip
  tip.show = function() {
    var args = Array.prototype.slice.call(arguments)
    if(args[args.length - 1] instanceof SVGElement) target = args.pop()

    var content = html.apply(this, args),
        poffset = offset.apply(this, args),
        dir     = direction.apply(this, args),
        nodel   = d3.select(node), i = 0,
        coords

    nodel.html(content)
      .style({ opacity: 1, 'pointer-events': 'all' })

    while(i--) nodel.classed(directions[i], false)
    coords = direction_callbacks.get(dir).apply(this)
    nodel.classed(dir, true).style({
      top: (coords.top +  poffset[0]) + 'px',
      left: (coords.left + poffset[1]) + 'px'
    })

    return tip
  }

  // Public - hide the tooltip
  //
  // Returns a tip
  tip.hide = function() {
    nodel = d3.select(node)
    nodel.style({ opacity: 0, 'pointer-events': 'none' })
    return tip
  }

  // Public: Proxy attr calls to the d3 tip container.  Sets or gets attribute value.
  //
  // n - name of the attribute
  // v - value of the attribute
  //
  // Returns tip or attribute value
  tip.attr = function(n, v) {
    if (arguments.length < 2 && typeof n === 'string') {
      return d3.select(node).attr(n)
    } else {
      var args =  Array.prototype.slice.call(arguments)
      d3.selection.prototype.attr.apply(d3.select(node), args)
    }

    return tip
  }

  // Public: Proxy style calls to the d3 tip container.  Sets or gets a style value.
  //
  // n - name of the property
  // v - value of the property
  //
  // Returns tip or style property value
  tip.style = function(n, v) {
    if (arguments.length < 2 && typeof n === 'string') {
      return d3.select(node).style(n)
    } else {
      var args =  Array.prototype.slice.call(arguments)
      d3.selection.prototype.style.apply(d3.select(node), args)
    }

    return tip
  }

  // Public: Set or get the direction of the tooltip
  //
  // v - One of n(north), s(south), e(east), or w(west), nw(northwest),
  //     sw(southwest), ne(northeast) or se(southeast)
  //
  // Returns tip or direction
  tip.direction = function(v) {
    if (!arguments.length) return direction
    direction = v == null ? v : d3.functor(v)

    return tip
  }

  // Public: Sets or gets the offset of the tip
  //
  // v - Array of [x, y] offset
  //
  // Returns offset or
  tip.offset = function(v) {
    if (!arguments.length) return offset
    offset = v == null ? v : d3.functor(v)

    return tip
  }

  // Public: sets or gets the html value of the tooltip
  //
  // v - String value of the tip
  //
  // Returns html value or tip
  tip.html = function(v) {
    if (!arguments.length) return html
    html = v == null ? v : d3.functor(v)

    return tip
  }

  function d3_tip_direction() { return 'n' }
  function d3_tip_offset() { return [0, 0] }
  function d3_tip_html() { return ' ' }

  var direction_callbacks = d3.map({
    n:  direction_n,
    s:  direction_s,
    e:  direction_e,
    w:  direction_w,
    nw: direction_nw,
    ne: direction_ne,
    sw: direction_sw,
    se: direction_se
  }),

  directions = direction_callbacks.keys()

  function direction_n() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.n.y - node.offsetHeight,
      left: bbox.n.x - node.offsetWidth / 2
    }
  }

  function direction_s() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.s.y,
      left: bbox.s.x - node.offsetWidth / 2
    }
  }

  function direction_e() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.e.y - node.offsetHeight / 2,
      left: bbox.e.x
    }
  }

  function direction_w() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.w.y - node.offsetHeight / 2,
      left: bbox.w.x - node.offsetWidth
    }
  }

  function direction_nw() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.nw.y - node.offsetHeight,
      left: bbox.nw.x - node.offsetWidth
    }
  }

  function direction_ne() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.ne.y - node.offsetHeight,
      left: bbox.ne.x
    }
  }

  function direction_sw() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.sw.y,
      left: bbox.sw.x - node.offsetWidth
    }
  }

  function direction_se() {
    var bbox = getScreenBBox()
    return {
      top:  bbox.se.y,
      left: bbox.e.x
    }
  }

  function initNode() {
    var node = d3.select(document.createElement('div'))
    node.style({
      position: 'absolute',
      opacity: 0,
      pointerEvents: 'none',
      boxSizing: 'border-box'
    })

    return node.node()
  }

  function getSVGNode(el) {
    el = el.node()
    if(el.tagName.toLowerCase() == 'svg')
      return el

    return el.ownerSVGElement
  }

  // Private - gets the screen coordinates of a shape
  //
  // Given a shape on the screen, will return an SVGPoint for the directions
  // n(north), s(south), e(east), w(west), ne(northeast), se(southeast), nw(northwest),
  // sw(southwest).
  //
  //    +-+-+
  //    |   |
  //    +   +
  //    |   |
  //    +-+-+
  //
  // Returns an Object {n, s, e, w, nw, sw, ne, se}
  function getScreenBBox() {
    var targetel   = target || d3.event.target,
        bbox       = {},
        matrix     = targetel.getScreenCTM(),
        tbbox      = targetel.getBBox(),
        width      = tbbox.width,
        height     = tbbox.height,
        x          = tbbox.x,
        y          = tbbox.y,
        scrollTop  = document.documentElement.scrollTop || document.body.scrollTop,
        scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft


    point.x = x + scrollLeft
    point.y = y + scrollTop
    bbox.nw = point.matrixTransform(matrix)
    point.x += width
    bbox.ne = point.matrixTransform(matrix)
    point.y += height
    bbox.se = point.matrixTransform(matrix)
    point.x -= width
    bbox.sw = point.matrixTransform(matrix)
    point.y -= height / 2
    bbox.w  = point.matrixTransform(matrix)
    point.x += width
    bbox.e = point.matrixTransform(matrix)
    point.x -= width / 2
    point.y -= height / 2
    bbox.n = point.matrixTransform(matrix)
    point.y += height
    bbox.s = point.matrixTransform(matrix)

    return bbox
  }

  return tip
};

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<head>
<link rel="stylesheet" type="text/css" href="barchart.css">
</head>
<body>
    <script src="https://d3js.org/d3.v3.min.js"></script>
    <script src="d3.tip.v0.6.3.js"></script>
<script>

var margin = {top: 40, right: 20, bottom: 30, left: 70},
    width = 460 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

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

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")
    .ticks(10, "");

var tip = d3.tip()
  .attr('class', 'd3-tip')
  .offset([-10, 0])
  .html(function(d) {
    return "<strong>Count:</strong> <span style='color:red'>" + d.Count + "</span>";
  })
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 + ")");

svg.call(tip);
d3.json("pottery.json", function(error, data) {

  if (error) throw error;

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

  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", -36)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Count");

  svg.selectAll(".bar")
      .data(data)
    .enter().append("rect")
      .attr("class", "bar")
      .attr("x", function(d) { return x(d.PotteryType); })
      .attr("width", x.rangeBand())
      .attr("y", function(d) { return y(d.Count); })
      .attr("height", function(d) { return height - y(d.Count); })
      .on('mouseover', tip.show)
      .on('mouseout', tip.hide);
});

function type(d) {
  d.Count = +d.Count;
  return d;
}

</script>

pottery.json

[
  {"PotteryType": "Terra Sig","Count": 58},
  {"PotteryType": "African Red Slip","Count": 81},
  {"PotteryTyp": "D24","Count": 39},
  {"PotteryType": "Vernice Nera","Count": 80},
  {"PotteryType": "Bipedalis Fragment","Count": 99}
]