用于Morris图数据的jQuery post的setInterval

I'm using jQuery to post and retrieve data from my controller (codeigniter). I've no problem with retrieving data. I want to use setInterval to update the morris data and make the morris data (JSON data) dynamic. The problem is when I use the setinterval to resend the update request to controller the graph disappear! I'm using the codeigniter output:

$this->output
->set_status_header(200)
->set_content_type('application/json', 'utf-8')
->set_output(json_encode($data1['query'], JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES))
->_display();
exit;

And this is the JavaScript code that I'm trying to use:

var json = (function () {
        var json = null;
        setInterval(function() {
        $.post( "/login/DailyAverageWaitingtime", function( data ) {
            json = data;
        });
        }, 5000);
        return json;
    })
    ();

    Morris.Area({
    element: 'DailyAverageWaitingtime',
    padding: 10,
    behaveLikeLine: true,
    gridEnabled: false,
    gridLineColor: '#dddddd',
    axes: true,
    fillOpacity:.7,
    data:json.result_object,
    lineColors:['#ED5D5D','#D6D23A','#32D2C9'],
    xkey: 'date',
    ykeys:['records','waiting'],
    labels: ['Records','Waiting'],

    pointSize: 0,
    lineWidth: 0,
    hideHover: 'auto'
});

Finally I fixed the issue!

This is a temporary function to create the graph at first place:

function DailyAverageWaitingtime(){
    var DailyAverageWaitingtime;
        $.post( "/login/DailyAverageWaitingtime", function( data ) {
            DailyAverageWaitingtime = Morris.Area({
                element: 'DailyAverageWaitingtime',
                data: data.result_object,
                xkey: 'date',
                ykeys: ['records', 'waiting'],
                labels: ['Records', 'Waiting'],
                pointSize: 2,
                hideHover: 'auto',
                resize: true
            });

Interval to call the update function:

            setInterval(function() { UpdateDailyAverageWaitingtime(DailyAverageWaitingtime); }, 20000);
        }, "json"); 
}

And this is to update the data every X second:

function UpdateDailyAverageWaitingtime(DailyAverageWaitingtime){
    $.post( "/login/DailyAverageWaitingtime", function( data ) {
        DailyAverageWaitingtime.setData(data.result_object);
        myRecords.setData(data.result_object);

            });
}

        var json = null;
        setInterval(function() {
        $.post( "/login/DailyAverageWaitingtime", function( data ) {
            json = data;
        });
        }, 5000);


    Morris.Area({
    element: 'DailyAverageWaitingtime',
    padding: 10,
    behaveLikeLine: true,
    gridEnabled: false,
    gridLineColor: '#dddddd',
    axes: true,
    fillOpacity:.7,
    data:json.result_object,
    lineColors:['#ED5D5D','#D6D23A','#32D2C9'],
    xkey: 'date',
    ykeys:['records','waiting'],
    labels: ['Records','Waiting'],

    pointSize: 0,
    lineWidth: 0,
    hideHover: 'auto'
});

Use different name for the variable which is containing function