绑定事件比较特殊：必须用 @click.native=‘’ 帮i的那个时间 否则无效
<el-dropdown>
<span class="el-dropdown-link" ref="echarType">
柱状图<i class="el-icon-arrow-down el-icon--right"></i>
</span>
<el-dropdown-menu slot="dropdown" >
<el-dropdown-item>柱状图</el-dropdown-item>
<el-dropdown-item @click.native="seeTable">表格</el-dropdown-item>
</el-dropdown-menu>
</el-dropdown>