Attach an annotation to an expression
Annotation[expr, data]
Annotation[expr, data, "type"]
Annotation[items, key -> value]
Cases[a, Annotation[_, True]]
→ {}Plot[{Annotation[Sin[x], "Sine", "Mouse"], Annotation[Cos[x], "Cosine", "Mouse"]}, {x, 0, 2π}, PlotStyle -> Thick]
→ -Graphics-