di·a·gram / ˈdīəˌgram/ • n. (abbr.: diag.) a simplified drawing showing the appearance, structure, or workings of something; a schematic representation: a diagram of the living room. ∎  Geom. a figure composed of lines that is used to illustrate a definition or statement or to aid in the proof of a proposition.• v. (-gramed, -gram·ing; also -grammed, -gram·ming) [tr.] represent (something) in graphic form: the experiment is diagramed on page fourteen.DERIVATIVES: di·a·gram·mat·ic / ˌdīəgrəˈmatik/ adj.di·a·gram·mat·i·cal·ly / ˌdīəgrəˈmatik(ə)lē/ adv.

diagram XVII. — L. diagramma — Gr. diágramma (-at-), f. diagráphein mark out by lines, f. DIA- + gráphein write; see -GRAM.
So diagrammatic XIX.