The black dots show the midpoint computed by d3-shape’s arc.centroid. Note that this is not the geometric center of the arc, which may be outside the arc; this method is merely a convenience for positioning labels.
<!DOCTYPE html>
<meta charset="utf-8">
<canvas width="960" height="500"></canvas>
<script src="//d3js.org/d3.v4.0.0-alpha.4.min.js"></script>
<script>
var data = [1, 1, 2, 3, 5, 8, 13];
var canvas = document.querySelector("canvas"),
context = canvas.getContext("2d");
var width = canvas.width,
height = canvas.height,
radius = Math.min(width, height) / 2;
var colors = [
"#1f77b4", "#ff7f0e", "#2ca02c", "#d62728", "#9467bd",
"#8c564b", "#e377c2", "#7f7f7f", "#bcbd22", "#17becf"
];
var arc = d3.arc()
.outerRadius(radius - 10)
.innerRadius(radius - 70)
.context(context);
var dot = d3.symbol()
.context(context);
var pie = d3.pie();
var arcs = pie(data);
context.translate(width / 2, height / 2);
context.globalAlpha = 0.5;
arcs.forEach(function(d, i) {
context.beginPath();
arc(d);
context.fillStyle = colors[i];
context.fill();
});
context.globalAlpha = 1;
context.beginPath();
arcs.forEach(arc);
context.lineWidth = 1.5;
context.stroke();
context.beginPath();
arcs.forEach(function(d) {
var c = arc.centroid(d);
context.save();
context.translate(c[0], c[1]);
dot();
context.restore();
});
context.fillStyle = "#000";
context.fill();
</script>