如何读取 git 日志图

Git 社区的书上说

您可以做的另一件有趣的事情是使用‘—— graph’选项可视化提交图，如下所示:

$ git log --pretty=format:'%h : %s' --graph
* 2d3acf9 : ignore errors from SIGCHLD on trap
*   5e3ee11 : Merge branch 'master' of git://github.com/dustin/grit
|\
| * 420eac9 : Added a method for getting the current branch.
* | 30e367c : timeout code and tests
* | 5a09431 : add timeout protection to grit
* | e1193f8 : support for heads with slashes in them
|/
* d6016bc : require time for xmlschema

It will give a pretty nice ASCII representation of the commit history lines.

我应该如何阅读这个图表? 420eac9与其他图表有什么不同？