Exactly one relation for OmniGraffle place file in ~/Library/Application Support/OmniGraffle/Shapes

ExactOne.graffleshapes

{
  arrowheads = (
		{
			Gap = 0.5;
			LineGap = 1.0;
			Name = ExactOne;
			Path = {
				elements = (
					{element = MOVETO; point = "{12, 6}"; },
					{element = CURVETO; control1 = "{3, 6}"; control2 = "{3, -6}"; point = "{12, -6}"; }, 
				);
			};
			Width = 7;
			ShouldExport = YES;
		},
	);
}