learning JSON

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
var spaceCircles = [30, 70, 110];
 
 var svgContainer = d3.select("body").append("svg")
                                     .attr("width", 2000)
                                    .attr("height", 200);

var circles = svgContainer.selectAll("circle")
                          .data(spaceCircles)
                          .enter()
                          .append("circle");

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js