oppressive vs unchallenging

oppressive

adj
  • Hot and humid of the weather. 

  • Tyrannical or exercising unjust power. 

  • Burdensome or difficult to bear. 

  • Weighing heavily on the spirit; intense, or overwhelming 

unchallenging

adj
  • Not challenging; easy to do. 

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