assign custom reduction value to variable - dc.js

Question

I calculate the average of specific column with below code:
var averageGroup = all.reduce(
function(p, v) {
++p.number;
p.StockDay += v.StockDay ;
p.average= p.StockDay/p.number ;
return p;
},
function(p, v) {
--p.number;
p.StockDay -= v.StockDay ;
p.average= p.StockDay/p.number ;
return p;
},
function() {
return {
number: 0,
average: 0,
StockDay : 0,
};
}
);
Now I want to assign this average value to variable because I will use it within my dc.barChart.
I want to assign different colors to columns where value is more than average.
The problem is when I make filter on charts it redraw the barchart but it doesn't change color according to new average which is calculated in averageGroup above.
var stockChart= dc.barChart('#stock')
.width(2000)
.height(600)
.margins({top: 10, right: 0, bottom: 130, left: 80})
.dimension(areaDim)
.group(areaGroup)
.valueAccessor(function(p) {
return p.value.avg;
})
.x(d3.scale.ordinal())
.xUnits(dc.units.ordinal)
.ordering(function(p) {return -p.value.avg})
.colors(d3.scale.ordinal().domain(["positive", "negative"])
.range(["#FF0000", "#00FF00"]))
.colorAccessor(function(p) {
if (p.value.avg> mean) {
return "positive";
}
return "negative";
})
.elasticX(true)
.renderlet(function (chart) {
chart.selectAll("g.x text")
.attr('dx', '-40')
.attr('transform', "rotate(-45)");
})
.elasticY(true)
.yAxis().tickFormat(d3.format('.3s'))
I calculate the mean variable shown below: But it gives me static value and doesn't change as I filter charts.
var selectedData = data.filter(function(d) {
return d.StockDay;
})
mean = d3.mean(selectedData,function(d) { return d.StockDay]})
But I want this mean variable come from averageGroup custom reduction function so when I filter charts and average changes my dc.barchart will change color according to the filtered average.