Arc Corners

In D3 3.5, you can generate rounded arcs with d3.svg.arc cornerRadius and separate adjacent arcs with d3.layout.pie padAngle. The schematic representation above shows the circles used to round the arcs’ corners.

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var data = [1, 1, 2, 3, 5, 8, 13];

var width = 960,
    height = 500,
    outerRadius = height / 2 - 10,
    innerRadius = outerRadius - 60,
    cornerRadius = 12,
    padAngle = .03;

var pie = d3.layout.pie()
    .padAngle(padAngle);

var arcs = pie(data);

var arc = d3.svg.arc()
    .innerRadius(innerRadius)
    .outerRadius(outerRadius)
    .cornerRadius(cornerRadius);

var color = d3.scale.category10();

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
  .append("g")
    .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

svg.append("circle")
    .style("fill", "none")
    .style("stroke", "#555")
    .attr("r", innerRadius);

svg.append("circle")
    .style("fill", "none")
    .style("stroke", "#555")
    .attr("r", outerRadius);

svg.append("g")
    .style("stroke", "#555")
    .style("fill", "none")
    .attr("class", "corner")
  .selectAll("circle")
    .data(d3.merge(arcs.map(function(d) {
      return [
        {angle: d.startAngle + padAngle / 2, radius: outerRadius - cornerRadius, start: +1},
        {angle:   d.endAngle - padAngle / 2, radius: outerRadius - cornerRadius, start: -1},
        {angle:   d.endAngle - padAngle / 2 * outerRadius / innerRadius, radius: innerRadius + cornerRadius, start: -1},
        {angle: d.startAngle + padAngle / 2 * outerRadius / innerRadius, radius: innerRadius + cornerRadius, start: +1}
      ];
    })))
  .enter().append("circle")
    .attr("cx", function(d) { return d.start * cornerRadius * Math.cos(d.angle) + Math.sqrt(d.radius * d.radius - cornerRadius * cornerRadius) * Math.sin(d.angle); })
    .attr("cy", function(d) { return d.start * cornerRadius * Math.sin(d.angle) - Math.sqrt(d.radius * d.radius - cornerRadius * cornerRadius) * Math.cos(d.angle); })
    .attr("r", cornerRadius);

svg.append("g").selectAll("path")
    .data(arcs)
  .enter().append("path")
    .style("fill", function(d, i) { return color(i); })
    .style("fill-opacity", .25)
    .style("stroke", "#000")
    .style("stroke-width", "1.5px")
    .attr("d", arc);

</script>

https://d3js.org/d3.v3.min.js