Group Rollup Function

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
   <link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
  <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% }
    
    .table th{
      text-align:center;
    } 
    
    .table td{
      text-align:center;
    } 
    .table td:first-child{
      
      text-align:left
    }
  </style>
</head>

var table = d3.select("#roster")
  .append("table")
  .classed("table", true);
var thead = table.append("thead");
var tbody = table.append("tbody");

//console.log(nested_data);

nested_data.forEach(function (d) {
    d.Period = months[d.key.split("-").pop() - 1] + "-" + d.key.slice(0,4) ;
    d.Visits = mformat(d.values.visits);
    d["Bounce Rate"] = pctformat(d.values.BounceRate);
    d["Avg. Visit Duration"] = r2precision(d.values.AVD)
});
  
 marketSelector.selectAll("option")
    .data(markets)
    .enter()
    .append("option")
    .attr("value", function(d) { return d; })
    .text(function(d) { return d; })
    .sort(function(a,b) { return d3.ascending(a,b); });
 
  thead.selectAll("tr")
    .data([columns])
    .enter()
    .append("tr")
    .selectAll("th")
    .data(function(d) { return d; })
    .enter()
    .append("th")
   .on("click",  function(d) {
      tbody.selectAll("tr")
        .sort(function(a,b) {
          return (d === 'No')
            ? d3.ascending(+a[d], +b[d])
            : d3.ascending(a[d], b[d])
        })
     .style("background-color", function(d,i) { return (i%2)? "white":"lightgray"; });
    }).text(function(d) { return d; })
    ;
 
var rows = tbody.selectAll("tr")
    .data(nested_data);
    
  rows.enter()
    .append("tr")
    .style("background-color", function(d,i) { return (i%2)? "white":"lightgray"; });
  rows.exit().remove();
  
  var cells = rows.selectAll("td")
    .data(function(row) { return columns.map(function(col) {
      return row[col];
    });});
    
  cells.enter().append("td");
  
  cells.text(function(d) { return d; });  
  
};

//end of redraw function

var selectMarket = function(marketId) {
  var roster = data.filter(function(d) { return d.Market === marketId; });
  redraw(roster);
  
 // d3.select("#market-name").text(teams[teamId] + " Roster");
 // document.getElementById("market-selector").value = teamId;  
  
};
reload();  
</script>
</body>

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