先来看看效果图，每个线条拐点使用不一样的图案
实现方式：
设置series里的symbol和symbolSize属性，echarts默认支持7种图案：circle，rect，roundRect，triangle，diamond，pin，arrow，symbolSize用于设置大小，[9，9] 表示symbol的宽为9，高为9。宽和高一样可以简写为一个数，如symbolSize：9。
如果想换其他图案，需要自己引入svg格式图片：复制svg的路径，
例：
//拐点图案
symbolList:['circle','rect','roundRect','triangle','diamond','pin','arrow',
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]
即可实现效果
