Past tense of diagram; represented or explained using a diagram (American spelling variant).
From diagram (Greek dia- 'through' + graphein 'to write') plus -ed past tense suffix. This is the American English single-m spelling; British English uses 'diagrammed' with double m.
English spelling varies because American and British English evolved different rules for doubling consonants—'diagramed' vs 'diagrammed' shows this perfectly. American English tends to prefer shorter spellings, while British English often keeps the doubled consonant to preserve the short vowel sound.
Complete word intelligence in one call. Free tier — 50 lookups/day.