绘制流程图，形成文档

可以使用的工具

• https://www.websequencediagrams.com/
• MY CHOICE: https://mermaidjs.github.io/
• Another choice: google key word: sequence graph

把 svg 转换成 png 文件

Or without installing anything:

It will produce picture.svg.png that is 1000 pixels wide.

I have tested it only on OS X 10.6.3.

例如：