different between digraphic vs diagraphic

English

digraph +? -ic

digraphic (not comparable)

Of or pertaining to a digraph.
- 1874, Henry Sweet, A History of English Sounds from the Earliest Period
  Cases of the arbitrary use of consonants as digraphic modifiers also occur

digraphic in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.

diagraphic (comparative more diagraphic, superlative most diagraphic)

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