Error to the popular Google Chart

Question

The following error appears

Invalid data table format: column #0 must be of type 'string'.

Right now I have the return in json this way:

[{"Field":"OrderTask","Old":"1","New":"2","Modified":"2016-12-08 11:16:27"},
{"Field":"Status","Old":"1","New":"4","Modified":"2016-12-08 13:43:51"}]

And I have the following Google Chart code:

$("#dataTables-tasks tbody").on("click", "tr", function(){

    var idTask = $(this).find('td.id-task').html();
    google.charts.load('current', {'packages':['gantt']});
    google.charts.setOnLoadCallback(drawChart);


     function drawChart() {
       var jsonData = $.ajax({
       url: "../connect/post/select.php",
       dataType: "json",
       data:'numIdTask='+idTask
     }).responseText;

      var data = new google.visualization.DataTable(jsonData);

      var options = {
        width: 550,
        height: 400,
        gantt: {
          trackHeight: 30
        }
      };

      var chart = new google.visualization.Gantt(document.getElementById('chart_div'));

      chart.draw(data, options);
    }
});