Adding Groups to SVG

<!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>

https://d3js.org/d3.v4.min.js