Of, relating to, or of the nature of a diagram; presented in diagrammatic form.
From diagrammatic plus the suffix -al, creating a doubly-suffixed adjective. This is a more formal or archaic variant of diagrammatic itself.
Adding -al to words already ending in -ic (like 'diagrammatical' or 'mathematical') is an older English pattern that sounds increasingly formal or old-fashioned. Modern writers prefer 'diagrammatic,' but dictionaries still recognize this longer variant.
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