pencil lead vs plumbago

pencil lead and plumbago both are nouns.

Word	Noun	Adjective	Verb	Adverb
pencil lead	Yes	No	No	No
plumbago	Yes	No	No	No

As nouns, plumbago is a hypernym of pencil lead; that is, plumbago is a word with a broader meaning than pencil lead:

pencil lead: mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil
plumbago: used as a lubricant and as a moderator in nuclear reactors

Other hypernyms of pencil lead include black lead, graphite.

pencil lead (noun)	plumbago (noun)
mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil	any plumbaginaceous plant of the genus Plumbago
	used as a lubricant and as a moderator in nuclear reactors