text: Title: why plot return is None?
  Body: ### System Info
  
   pandasai==1.5.7, python==3.9
  
  ### 🐛 Describe the bug
  
  hi,
  when i ran
  ```py
  g = df.chat('plot hist for country')
  ```
  
  `g` is `None` instead of a pd plot or pyplot. Is this desired behaviour?
  
  thanks