[рони]

<script>
 var data = [{
        Fio: "Ivanov",
        Date: "January 16",
        Parametr: "200"
    }, {
        Fio: "Petrov",
        Date: "January 16",
        Parametr: "300"
    }, {
        Fio: "Ivanov",
        Date: "February 16",
        Parametr: "100"
    }, {
        Fio: "Petrov",
        Date: "February 16",
        Parametr: "150"
    }, {
        Fio: "Ivanov",
        Date: "March 16",
        Parametr: "400"
    }, {
        Fio: "Petrov",
        Date: "March 16",
        Parametr: "500"
    }, {
        Fio: "Sidorov",
        Date: "March 16",
        Parametr: "300"
    }],
    obj = {},
    data = data.reduce(function(b, c) {
        var a = c.Date.split(" ")[0];
        void 0 === obj[a] && (obj[a] = b.length, b[b.length] = {Month: a});
        a = obj[a];
        b[a][c.Fio] = c.Parametr;
        return b
    }, []);

document.write(JSON.stringify(data))
  </script>