Past tense of diagram; represented or explained using a diagram (British spelling variant with doubled m).
From diagram plus -med (past tense), with doubled m following British conventions for verbs with short vowels. This is standard in British English.
The battle between 'diagrammed' (British) and 'diagramed' (American) reveals that English spelling conventions aren't universal truths but rather regional choices. British and American English speakers are equally 'correct'—they just follow different rule books that formed historically.
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