[ext 3.0.3] Pie chart disappear wher chart store is reloaded

Hi all!
i'm playing with charts and particularly with Pie chart.
I've created a task that , every 60 seconds, reload chart pie store.
First time, chart loads succesfully , but when store is reloaded , graphs is not rendered , what i get you can see in images attached
Does anyone have solved this problem , or has any solution ?
Thanks in advance

Code:

---

    var task = {
        run: function(){
            store.reload();
        },
        interval: 60000 //1 second
    }
    Ext.TaskMgr.start(task);
var chartPie = new Ext.chart.PieChart({
                        store : store,
                        dataField: 'totale',
                        categoryField: 'desc',
                        extraStyle:
            {
                legend:
                {
                    display: 'bottom',
                    padding: 5,
                    font:
                    {
                        family: 'Tahoma',
                        size: 13
                    }
                }
            }

                });

---