Marco,
What is going wrong is that you are assigning a new array to dps while dataPoints is still pointing to old one. So instead you should either modify existing dps array or assign a totally new array to dataPoints.
1st Approach:
var updateChart = function () {
    $.getJSON(“data.php”, function (result) {
        dps.splice(0, dps.length);
        $.each(result, function (index, value) {
            dps.push(value);
        });
    });
    chart.render();
};
2nd Approach:
var updateChart = function () {
    $.getJSON(“data.php”, function (result) {
        chart.options.data[0].dataPoints = result;
    });
    chart.render();
};