https://stackoverflow.com/questions/22149584/what-does-axis-in-pandas-mean?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
https://www.jianshu.com/p/9aa448ea397c
http://frankchen.xyz/2017/12/12/Understanding-the-axis-parameter-in-Pandas-and-Numpy/
