echarts4.0.2里myChart.on()不能获取categories的返回

[tudou167]

One-line summary [问题简述]

echarts4.0.2里myChart.on()不能获取顶部分类的返回 线与节点皆没有问题 console.log(myChart) 里找不到顶部分类的对象

Version & Environment [版本及环境]

• ECharts version [echarts4.0.2]:
• Browser version [Google Chrome 版本 63.0.3239.108（正式版本） （32 位）]:
• OS Version [win7 64位]:

Expected behaviour [期望结果]

myChart.on() 能触发分类并返回参数 或者能获取对象绑定事件

ECharts option [ECharts配置项]

    categories: [{
        name: periods[0],
        itemStyle: {
            normal: {
                color: periodsColor[0],
            }
        }
    }, {
        name: periods[1],
        itemStyle: {
            normal: {
                color: periodsColor[1],
            }
        }
    }, {
        name: periods[2],
        itemStyle: {
            normal: {
                color: periodsColor[2],
            }
        }
    }, {
        name: '测试',
        itemStyle: {
            normal: {
                color: periodsColor[2],
            }
        }
    }],

### Other comments [其他信息]
<!-- For example: Screenshot or Online demo -->
<!-- [例如，截图或线上实例 (JSFiddle/JSBin/Codepen)] -->

myChart.on('click', function (params) {
        //console.log(myChart);
        //console.log(params);

    });

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