As diction only refers to spoken word, I'll have to correct you on that.
you might want to tell that to several dictionaries...
1. style of speaking or writing as dependent upon choice of words: good diction.
2. the accent, inflection, intonation, and speech-sound quality manifested by an individual speaker, usually judged in terms of prevailing standards of acceptability; enunciation.
1400–50; late ME diccion < LL dictio-n- (s. of dictio-) word, L: rhetorical delivery, equiv. to dict(us) said, spoken (ptp. of di-cere) + -io-n- -ion
Based on the Random House Dictionary, © Random House, Inc. 2009
1. Choice and use of words in speech or writing.
2. Degree of clarity and distinctness of pronunciation in speech or singing; enunciation.
[Middle English diccion, a saying, word, from Old French, from Latin dictio-, dictio-n-, rhetorical delivery, from dictus, past participle of di-cere, to say, speak; see deik- in Indo-European roots.]
dic'tion·al adj., dic'tion·al·ly adv.
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2006 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
notice the primary meaning in both entries. but etymologically speaking, you are correct...
1542, from L.L. dictionem (nom. dictio), from L. "a saying, expression, word," from dic-, stem of dicere "speak, tell, say," related to dicare "proclaim, dedicate," from PIE base *deik- "to point out" (cf. Skt. dic- "point out, show," Gk. deiknynai "to prove," O.H.G. zeigon, Ger. zeigen "to show," O.E. teon "to accuse," tæcan "to teach").
Online Etymology Dictionary, © 2001 Douglas Harper