let down vs wallow in the mire

let down

verb
  • Used other than figuratively or idiomatically: see let, down. 

  • To disappoint; to betray or fail somebody 

  • To reduce one's level of effort. 

  • To soften in tempering. 

  • To allow to descend. 

  • To lengthen by undoing and resewing a hem. 

wallow in the mire

verb
  • Used other than figuratively or idiomatically: see wallow, mire. 

  • To revel in baseness or scandal. 

How often have the words let down and wallow in the mire occurred in a corpus of books? (source: Google Ngram Viewer )