unique value reduction

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/crossfilter/1.3.9/crossfilter.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.12/d3.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/dc/2.1.8/dc.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/dc/2.1.8/dc.css">
<div id="ntotal"></div>

JavaScript

data = [
{"Respond_Id":258,"Gender":"Female","Age":"18-21","Answer":424},
{"Respond_Id":258,"Gender":"Female","Age":"18-21","Answer":428},
{"Respond_Id":261,"Gender":"Male","Age":"22-26", "Answer":427},
{"Respond_Id":261,"Gender":"Male","Age":"22-26",  "Answer":432},
{"Respond_Id":345,"Gender":"Female","Age":"27-30","Answer":424},
{"Respond_Id":345,"Gender":"Female","Age":"27-30","Answer":425},
{"Respond_Id":345,"Gender":"Female","Age":"27-30","Answer":433}]

var ndx = crossfilter(data);

var RespondDim       =  ndx.dimension(function (d) { return true;});
var ntotal           =  RespondDim.group().reduce(
                                function(p, d) { 
                                    if(d.Respond_Id in p.Respond_Ids){
                                        p.Respond_Ids[d.Respond_Id]++;
                                    }
                                    else {
                                        p.Respond_Ids[d.Respond_Id] = 1;
                                        p.RespondCount++;
                                    }
                                    return p;
                                },
                                function (p, d) {
                                    p.Respond_Ids[d.Respond_Id]--;
                                    if(p.Respond_Ids[d.Respond_Id] === 0){
                                        delete p.Respond_Ids[d.Respond_Id];
                                        p.RespondCount--;
                                    }
                                    return p;
                                },
                                function () {
                                    return {
                                        RespondCount: 0,
                                        Respond_Ids: {}
                                    };
                                 });
var numberDisplay    = dc.numberDisplay("#ntotal");
numberDisplay
             .group(ntotal)
             .valueAccessor(function(d){...