chard vs leaf vegetable

chard

noun
  • An edible leafy vegetable, Beta vulgaris subsp. cicla, with a slightly bitter taste. 

  • Artichoke leaves and shoots, blanched to eat. 

leaf vegetable

noun
  • The leaf of a plant used as an edible vegetable, such as lettuce, spinach, or rocket. 

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