sit down vs straighten up

sit down

verb
  • To assume a sitting position from a standing position. 

  • To assume a low or sunken position. 

  • To meet formally at a conference table. 

  • To cause to be seated or in a sitting posture; to furnish a seat to. 

straighten up

verb
  • to sit up straight, to stop hunching. 

  • to start living a reformed life 

  • to steer straight 

  • to tidy; tidy up 

  • to deal with; put in order 

  • To become straight, or straighter 

  • To clarify 

How often have the words sit down and straighten up occurred in a corpus of books? (source: Google Ngram Viewer )