This appears to be a non-standard or erroneous formation; possibly an intended variant of 'diagrammatically.'
This word does not appear in standard English dictionaries and seems to be a misspelling or confusion of diagrammatically. The -mitic- sequence is not standard in English word formation.
This word shows the limits of even productive English patterns—you can't just stack any suffix onto any root and expect a real word. English has rules and traditions, and 'diagrammitically' violates them, suggesting it's either a typo or a non-word.
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