為echart添加水印

 graphic: [
                     {
                                     type: 'text',
                                      silent:true,
                                     z: 100,
                                     left: 'center',
                                     top: 'middle',
                                     style: {
                                         fill: '#ffffff20',
                                         text: [
                                             '© 2020 版權信息'
                                         ].join('\n'),
                                         fontSize:14
                                     }
                                 }
             ],