Trying to use Chart using data from SQLServer to json in php show blank

<div id="chart-container">
    <canvas id="graphCanvas"></canvas>
</div>
<script>
    $(document).ready(function () {
        showGraph();
    });
    function showGraph()
    {
        {
            $.post("grafik1.php",
            function (data)
            {
                console.log(data);
                 var name = [];
                var marks = [];

                for (var i in data) {
                    name.push(data[i].nama);
                    marks.push(data[i].jumlah_warga);
                }

                var chartdata = {
                    labels: name,
                    datasets: [
                        {
                            label: 'Nama Kota',
                            backgroundColor: '#49e2ff',
                            borderColor: '#46d5f1',
                            hoverBackgroundColor: '#CCCCCC',
                            hoverBorderColor: '#666666',
                            data: marks`enter code here`
                        }
                    ]
                };

                var graphTarget = $("#graphCanvas");

                var barGraph = new Chart(graphTarget, {
                    type: 'bar',
                    data: chartdata
                });
            });
        }
    }
    </script>
    <p>
  </p>
 </div>

code above are i used to call json data from grafik.php here below code for grafik1.php

<?php
  header('Content-Type: application/json');
  include "conf/conn.php";
  $sql = "SELECT nama,jumlah_warga FROM kota";
  $stmt = sqlsrv_query( $conn, $sql );

    if( $stmt === false)
{
    echo "Error in query preparation/execution.\n";  
    die( print_r( sqlsrv_errors(), true));  
}
    $data = array();
    while( $row = sqlsrv_fetch_array( $stmt, SQLSRV_FETCH_ASSOC))
{    
    $data[] = $row;
}
   echo json_encode($data);
   ?> 

And it show blank when im using the header,but when im not using it, its just show the graph without value or undefined, like the json was empty. what i want is, parsing select from sql server into json and call it via ajax on my index. please help