highchart, TypeError: obj is null

我想用PHP从html请求一个日期时间变量，

，并使用该变量从mysql数据库中选择数据。然后创建一个json来生成highchart。

成功生成json，但没有生成highchart.

我用Firebug它显示TypeError: obj is null

这是我的代码
PHP

<?php
   header('Content-type: application/json');
   $con = mysql_connect("localhost","root","");
   mysql_select_db("project", $con);
    $month=$_REQUEST['select_month'];
   $sqlChart_Q="SELECT T,value FROM `Temperature`WHERE month(T)='$month'";
   $sqlChart_R=mysql_query($sqlChart_Q);
   while($arrChart_ROW=mysql_fetch_array($sqlChart_R)) {
        $time=strtotime($arrChart_ROW[0])*1000;
          $temp=(float)$arrChart_ROW[1];     
   $arrChart[]=array("time" => $time,"value" => $temp);
    }
    echo json_encode($arrChart);
     ?>

和js

     var chart;
     $(document).ready(function() {           
     var options = {
        chart: {
           renderTo: 'month',
           type: 'spline',
        },
        title: {
        },
        xAxis: {
           type: 'datetime'
        },
        yAxis: {
        },
        series: [{
           name: 'Temperature',
           data: [] 
        }] 
        };
        $.getJSON('PHP/db_month.php', function(json) {
        temp = [];
        $.each(json, function(key,value) {
        temp.push([value.time,value.value]);
              });         
        options.series[0].data = temp;           
        chart = new Highcharts.Chart(options);
        });
        });

json (select_month=5;月美元= $ _REQUEST [' select_month '])

[{"time":1400076317000,"value":50},{"time":1400076322000,"value":25},{"time":1400076327000,"value":34},{"time":1400110911000,"value":50},{"time":1400110916000,"value":43},{"time":1400110919000,"value":75},{"time":1400110920000,"value":35},{"time":1400110922000,"value":46},{"time":1400110924000,"value":66},{"time":1400110925000,"value":73},{"time":1400113566000,"value":20},{"time":1400161118000,"value":55},{"time":1400186496000,"value":50},{"time":1400193165000,"value":43},{"time":1400196532000,"value":50}]

顺便说一下，当sql函数改为 时

    SELECT T,value FROM `Temperature`WHERE month(T)='5'

它将成功生成数据在五月创建的highchart。

(PS: T的类型是时间戳，我不知道是不是很重要。

我的代码有什么问题?有人能帮我解决这个问题吗?非常感谢!!

$arrChart = array();

$sqlChart_Q = "SELECT T,value FROM `Temperature`WHERE month(T)=" . (int) $month;