r.circle(100, 100, 50).attr({
    fill: "CadetBlue",
    stroke: "black"
}).mouseover(function (e) {
    var httpRequest = new XMLHttpRequest();
    httpRequest.onreadystatechange = function() {
        if (httpRequest.readyState === 4) {
            if (httpRequest.status === 200) {
                var msg = JSON.parse(httpRequest.responseText);

                // do stuff, for example show a popup

            } else {

                // fail

            }
        }
    };
    httpRequest.open('GET', url);
    httpRequest.send();
});