index.html
<!DOCTYPE html>
<head>
<meta charset="utf-8">
</head>
<style>
.viz-bar .bar{
fill: steelblue;
}
#labels text{
text-anchor:middle;
font-size:26px;
fill:white;
}
</style>
<body>
<svg width="960" height="500">
<g transform="translate(50,50)">
<g id="bars" ></g>
<g id="labels" ></g>
</g>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="//vizjs.org/viz.v1.3.0.min.js"></script>
<script>
var data=[{key:2, value:10},{key:3, value:4},{key:5, value:6},{key:8, value:1}];
var bar = viz.bar().data(data)
d3.select("#bars").call(bar);
d3.select("#labels")
.selectAll("text")
.data(bar.bars())
.enter()
.append("text")
.attr("dy","6")
.attr("x",function(d){ return d.x+d.width/2})
.attr("y",function(d){ return d.y+d.height/2})
.text(function(d){ return d.data.value; })
</script>