i3.js

// map 

var monthNames = ["January", "February", "March", "April", "May", "June",
  "July", "August", "September", "October", "November", "December"
];

map("Orders", o => {
    var results = [];
    for(var i = 0; i < o.Lines.length; i++){
        var cur = {
            Product: o.Lines[i].Product,
            Year: new Date(o.OrderedAt).getFullYear(),
        };
        var month = monthNames[new Date(o.OrderedAt).getMonth()];
        cur[month + " Sales"] = o.Lines[i].Quantity;
        cur[month + " Totals"] = o.Lines[i].Quantity * o.Lines[i].PricePerUnit * (1-o.Lines[i].Discount)
        results.push(cur);
    }
    return results;
})

// reduce

groupBy(x => ({Product: x.Product, Year: x.Year}))
    .aggregate(g=>{
        var final = {};
        for(var i = 0; i < g.values.length; i++) {
            var keys = Object.keys(g.values[i]);
            for(var j = 0; j < keys.length; j++) {
                if (keys[j].endsWith("Sales") == false && keys[j].endsWith("Totals") == false)
                    continue;
                final[keys[j]] = (final[keys[j]] || 0) + g.values[i][keys[j]];
            }
        }
        final.Product = g.key.Product;
        final.Year = g.key.Year;
        return final;
    });