projection debugging by fil

index.html

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v5.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <canvas width=960 height=500>
  <script>
    let width = 960;
    let height = 500;
    let context = d3.select("canvas").node().getContext('2d');
    let path1 = d3.geoPath();
    let path2 = d3.geoPath();

    const degrees = 180/Math.PI;

    let transform = (lng, lat) => {
      let x = d3.scaleLinear().domain([-180, 180]).range([0, width])(lng);
      let y = d3.scaleLinear().domain([-90, 90]).range([height, 0])(lat);
      return {x, y: y + x * 0.2
}
    };
    
    let projection2 = d3.geoTransform({
      point: function(x,y) {
        const t = transform(x, y);
        this.stream.point(t.x, t.y);
      }
    });
    
    let projection1 = d3.geoProjection((x,y) => {
      const t = transform(x*degrees, y*degrees);
      return [t.x, -t.y];
    }).scale(1)
    
    d3.json("world_countries.json").then(d => {
      path1.projection(projection1).context(context);

      context.strokeStyle = "#F00";
      context.beginPath();
      path1(d)
      context.stroke();
      
      path2.projection(projection2).context(context);

      context.strokeStyle = "#00F";
      context.beginPath();
      path2(d)
      context.stroke();
    })
  </script>
</body>