plonk oneself down vs sit down

plonk oneself down

sit down

verb
  • 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. 

  • To assume a sitting position from a standing position. 

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