plural schemata, 1796, in Kantian philosophy ("a product of the imagination intermediary between an image and a concept"), from Greek skhēma "figure, appearance, the nature of a thing," which is related to skhein "to get," and ekhein "to have, hold; be in a given state or condition" (from PIE root *segh- "to hold"). The meaning "diagrammatic representation" is from 1890; the general sense of "hypothetical outline" is by 1939.
1610s, "an illustrative figure giving only the outlines or general scheme of the object;" 1640s in geometry, "a drawing for the purpose of demonstrating the properties of a figure;" from French diagramme, from Latin diagramma "a scale, a musical scale," from Greek diagramma "geometric figure, that which is marked out by lines," from diagraphein "mark out by lines, delineate," from dia "across, through" (see dia-) + graphein "write, mark, draw" (see -graphy). Related: Diagrammatic; diagrammatically.
The verb, "to draw or put in the form of a diagram," is by 1822, from the noun. Related: Diagrammed; diagramming.