D3 Examples - Adding Groups to SVG

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>
​
<body>
  <script>
  var svg = d3.select("body").append("svg") 
    .attr("width", 960)
    .attr("height", 500)
  
  d3.json("data.json", function(error, json)  {
    var group = d3.select("svg").selectAll("g")
        .data(json)
        .enter()
        .append("g")
//here we will position the group using transformation matrices so that the rectangles and text get positioned together. 
      .attr ("transform" , function (d) {if (d.orientation === 'forward'){return "translate(" + d.start + ",100)";} else {return "translate(" + d.start + ",175)"; }});
    group.append("rect")
        .attr("height", 50)
        .attr("width", function (d) {return d.stop - d.start;})
        .attr("fill", function(d) {if (d.orientation == "forward") {return "green";  }
            else if (d.orientation == "reverse") {return "red";}    })
        .style("stroke", "black")
            .style("stroke-width", 3)
        group.append("text")
        .attr("x", function (d) {return (d.stop - d.start)/2 })
        .attr("y", 25)
        .text(function (d) {return d.name})
        .attr("text-anchor", "middle")
        .attr("alignment-baseline", "central");});
    
  </script>
</body>
​
    