1
general::
Also, artistic license. The liberty taken by a writer or artist in deviating from conventional form or fact to achieve an effect. For example, I've never seen grass or a tree of that color, but that's artistic license. [Late 1700s]
American Heritage Idioms
2
general::
Phrase(s): poetic license
liberties or license of the type taken by artists, especially poets, to violate patterns of rhyme, harmony, structure, etc. • I couldn’t tell whether he kept making spelling mistakes or if it was just poetic license.
McGrawhill's American Idioms And Phrasal Verbs