-gram1

1.
a combining form occurring in loanwords from Greek, where it meant “something written,” “drawing” (epigram; diagram); on this model, used in the formation of compound words (oscillogram).
Compare -graph.
Origin
< Greek -gramma, combining form of grámma something written or drawn; akin to carve

-gram2

1.
a combining form of gram1 :
kilogram.

-gram3

1.
a combining form extracted from telegram, used in the formation of compound words that have the general sense “message, bulletin”:
culturegram; electiongram; prophecy-gram.
British Dictionary definitions for -gram

-gram

combining form
1.
indicating a drawing or something written or recorded: hexagram, telegram
Word Origin
from Latin -gramma, from Greek, from gramma letter and grammē line
Word Origin and History for -gram

suffix from telegram (1852), first abstracted 1979 (in Gorillagram, a proprietary name in U.S.), and put to wide use in forming new words, such as stripagram (1981). The construction violates Greek grammar, as an adverb could not properly form part of a compound noun.

-gram in Medicine

-gram suff.
Something written or drawn; a record: cardiogram.