D3 Examples - Orthographic 'arc' using d3.graticule()

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<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="https://d3js.org/topojson.v1.min.js"></script>
<style>
.land {
  fill: #B6C598;
  stroke: #818141;
}
.arc {
  fill: red;
  fill-opacity: 0.3;
  stroke: black;
  stroke-opacity: 0.5;
}
</style>
<script>
​
var width = 960,
    height = 500,
    bb = {W: -5.0, N: 50.0, E: 10.0, S: 40.0 };
​
var projection = d3.geo.orthographic()
    .scale(250)
    .translate([width / 2, height / 2])
    .clipAngle(90);
​
var path = d3.geo.path()
    .projection(projection);
​
var arc = d3.geo.graticule()
    .majorExtent([[bb.W, bb.S], [bb.E, bb.N]])
​
var λ = d3.scale.linear()
    .domain([0, width])
    .range([-180, 180]);
​
var φ = d3.scale.linear()
    .domain([0, height])
    .range([90, -90]);
​
var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);
​
svg.on("mousemove", function() {
  var p = d3.mouse(this);
  projection.rotate([λ(p[0]), φ(p[1])]);
  svg.selectAll("path.land").attr("d", path);
  svg.selectAll("path.arc").attr("d", path(arc.outline()));
});
​
d3.json("world-110m.json", function(error, world) {
  svg.append("path")
      .datum(topojson.feature(world, world.objects.land))
      .attr("class", "land")
      .attr("d", path);
  svg.append("path")
      .attr("class", "arc")
      .attr("d", path(arc.outline()));
});
​
</script>
​
    