The phrase "excellent illustration" refers to a very good or impressive picture or drawing that helps to explain or portray something clearly. Full definition
This book contains excellent illustrations including toys, outdoor equipment, and a dog which I have found my own children really enjoyed looking at. (momentsaday.com)
Excellent illustration of the compounding effect of dividend increases. (dividendmantra.com)
Stoat often provides excellent illustrations of this. (rabett.blogspot.ru)