echarts 数据重新加载,原数据仍然存在图表上

echarts 数据重新加载,原数据依然存在图表上
利用AJAX加载数据以后,页面有一个搜索按钮,选择条件点击搜索,重新给xAxis.data和series.data重新赋值  然后myChar.SetOption(option)  ,为什么原来的数据还在图表上? 怎么去掉原来的数据?

if (data != null && data != "" && data != undefined) {

                                var json_list = eval('(' + data + ')');

                                var douhao = null;

                                var signedDate = "";

                                var count = "";

                                for (i = 0; i < json_list.length; i++) {

                                    if (i != json_list.length - 1) {

                                        douhao = ",";

                                    } else {

                                        douhao = "";

                                    }

                                    signedDate += json_list[i].SignedDate + douhao;

                                    count += json_list[i].Count + douhao;

                                }


                                $("#div_show_list").html(html);

                                var obj = signedDate.split(',');

                                var mon = count.split(',');

                                alert(obj + "," + mon);

                                option.xAxis[0].data = obj;

                                option.series[0].data = mon;

                                barChart.hideLoading();

                                barChart.setOption(option);

                            }

这是AJAX里面的一段代码,这段代码就是重新给myChar赋值.
------解决思路----------------------
文档里面有说：barChart.setOption(option, true); 就可以了
“万能接口，配置图表实例任何可配置选项（详见option），多次调用时option选项默认是合并（merge）的”