Request data from JavaScript via Flask RESTful API

My API sends data in the following form:

{
  "2015-09-05 00:00:00+000000": 0, 
  "2015-09-06 00:00:00+000000": 2, 
  "2015-09-07 00:00:00+000000": 1
}

What I would like to do is read the data with AJAX (?) and store them in two separate variables.

val a = ['a', '2015-09-05', '2015-09-06', '2015-09-07']
val b = ['b', 0, 2, 1]

I am new in JavaScript and AJAX but this is what I have done so far:

    function GetData() {

        // 1. Instantiate XHR - Start 
        var xhr; 
        if (window.XMLHttpRequest) 
            xhr = new XMLHttpRequest(); 
        else if (window.ActiveXObject) 
            xhr = new ActiveXObject("Msxml2.XMLHTTP");
        else 
            throw new Error("Ajax is not supported by your browser");

        // 3. Specify your action, location and Send to the server - End
        xhr.open('GET', 'http://localhost:5000/generate-aggregations');
        xhr.send(null);

    }


    var data = JSON.parse(GetData());

    var array_keys = new Array();
    array_keys.push('a');
    var array_values = new Array();
    array_values.push('b');

    for (var key in data) {
        array_keys.push(key);
        array_values.push(data[key]);
    }