綁定事件比較特殊：必須用 @click.native=‘’ 幫i的那個時間 否則無效
<el-dropdown>
<span class="el-dropdown-link" ref="echarType">
柱狀圖<i class="el-icon-arrow-down el-icon--right"></i>
</span>
<el-dropdown-menu slot="dropdown" >
<el-dropdown-item>柱狀圖</el-dropdown-item>
<el-dropdown-item @click.native="seeTable">表格</el-dropdown-item>
</el-dropdown-menu>
</el-dropdown>