一起使用HIghchart 与EXTJS 抛出错误

一起使用HIghchart 与EXTJS 抛出异常
界面是这样的界面最大的容器是一个EXTJS 的panel  
panel里面是一个tab
我想把highchart放到一个tab中的时候报错了
 

异常信息
uncaught exception: Highcharts error #13:  

                    var tabs = new Ext.TabPanel({

                        renderTo: strspot[a] + '-body',

                        activeTab: 0,

                        tabCls: 'tab-class',

                        items: [{

                            title: '实时数据',

                            html: html,

                        }, {

                            title: '实时曲线',

                            html: '<div id=chart' + a + ' class=tab_kid></div>'

                        }]

                    });



                    var chart = new Highcharts.Chart({

                        chart: {

                            renderto: 'chart' + a,

                            type: 'spline',