Just a little helper function. Depends on, and hooks into, underscore.

rolling-average.js

(function () {
    var root = this;
    
    var sum = function (arr) { 
        return _.reduce(arr, function (m, x) {
            return m + x;
        }); 
    };
    
    // n: Size of the stack, i.e., how far to look back 
    var rollingAverage = function (arr, n) {
        var stack = [];
        return _.map(arr, function (item) {
            stack.push(item);
            if (stack.length > n) { stack.shift(); }
            return sum(stack) / stack.length;
        });
    };
    
    root._.rollingAverage = rollingAverage;
}).call(this);