Counter

<!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; }
    svg { width:100%; height: 100% }
    
    circle {
      opacity: 0.5;
    }
  </style>
</head>

<body>
  <script>
    // Feel free to change or delete any of the code you see!
    var svg = d3.select("body").append("svg")
    
    var url = "https://numeracy.co/projects/382368K6Bkx/data?format=csv";
    
    d3.csv(url)
      .row(function(d){
      	return {
         name: d.name,
         age: parseFloat(d.age),
         date: new Date(d.month + "/" + d.day + "/" + d.year) 
        }
      })
      .get(function(err, data){
        console.log("data:", data);

var counted=[];
        var totalAge = 0;

data.forEach(function(d){
  //        console.log(d.age);
          if(d.age != "unknown" && !isNaN(d.age)){
            counted.push(d);
            totalAge += d.age;
          }
        })
      
        var maxAge = d3.max(counted, function(d){
          return d.age     
        });

var avgAge = d3.mean(counted, function(d){
          return d.age     
        });
      
      	console.log("mean age", avgAge);

var yscale = d3.scale.linear()
        .domain([0, maxAge])
        .range([400, 10]);

var dateExtent = d3.extent(counted, function(d) {
        return d.date
      })
      
      var xscale = d3.time.scale()
        .domain(dateExtent)
        .range([10, 900])

var circles=svg.selectAll("circle").data(counted)

circles.enter()
          .append("circle")
          .attr({
            cx: function(d,i) { 
              return xscale(d.date)
            } ,
            cy: function(d,i) {
              return yscale(d.age)
            },
            r:5
          })
          .on("click", function(d){
         	 	console.log("clicked",d)
        	})
    })
    console.log("you are now rocking with d3", d3);
  </script>
</body>

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