dataset could not be updated when jsonmodel changed

Hi, all. Please have a look on codes above,

var oModel2 = new sap.ui.model.json.JSONModel(aValues);

    var oDataset = new sap.viz.core.FlattenedDataset("dataSetOfLineChart", {

     dimensions : [ {

      axis : 1,

      name : "Key",

      value : (aValues[0].date == undefined)?"{key}":"{date}"

     } ],

     measures : [ {

      name : "Value",

      value : {"value"}

     }],

     data : {

      path : "/"

     }

    });

    oDataset.setModel(oModel2);

    oLineChart.setDataset(oDataset);

I get aValues from a json using ajax (async: false). When aValues changes, oDataset and the view of oLineChart don't change at the same time.

But when I request one more time, it works. I don't know how this happens. Could anyone please explain it to me?

Thanks!