sporadic vs unfrequent

sporadic

adj
  • Rare and scattered in occurrence. 

  • Exhibiting random behavior; patternless. 

unfrequent

adj
  • Not frequent; not common; not happening often; infrequent. 

verb
  • To stop frequenting; cease to frequent. 

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