swaying vs undulation

swaying

noun
  • The motion of something that sways. 

  • An injury caused to a horse's back by violent strains or overloading. 

undulation

noun
  • A wavelike motion of the air; electromagnetic radiation. 

  • An instance or act of undulating. 

  • A wavy appearance or outline; waviness. 

  • A wavelike curve; a smooth and regular rise and fall. 

  • A tremulous tone produced by a peculiar pressure of the finger on a string. 

How often have the words swaying and undulation occurred in a corpus of books? (source: Google Ngram Viewer )