title: Arrange row in specific order
body: <p>I have a data frame index (Grade) below, and it is sorted by alphabetical order. Is there a way I can decide on the arrangement such that A to H is above, followed by '2-6 mths' and '1 mth'? For columns, I know it is easy simply by <code>df=df[[col1,col2]]</code></p>
  
  <p><a href="https://i.stack.imgur.com/l5pH6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/l5pH6.png" alt="enter image description here"></a></p>