In a diagrammatic manner; in the form of or by means of a diagram.
From diagrammatic plus the adverb-forming suffix -ally. This builds on the adjective to create a manner adverb.
The chain diagrammatic → diagrammatical → diagrammatically shows how English creates adverbs by stacking suffixes, each one adding a layer of grammatical information. It's efficient but can sound unwieldy, which is why speakers often prefer 'in diagram form' instead.
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