A telegraph is a device or system for sending written messages over long distances. The technology is called telegraphy, and the printed message itself is a telegram.
If the logic of Greek grammar were to be followed, this kind of relationship would hold in general for compounds in which the final element is derived from the Greek verb γράφειν meaning ‘write.’ But it doesn’t. In the technology called photography, it’s the printed result that’s called a photograph, not the device (which is called a camera).
An epigraph is an inscription, or (in literature) a motto or quotation at the beginning of a book or chapter; an epigram is a short, witty saying or poem.
A radiograph is an x-ray photograph; a radiogram is (in American English) a telegram sent by radio.
A pentagram (or pentacle) is a five-pointed star. The word pentagraph is not found in most dictionaries, but it’s found in Wikipedia and means, by extension from digraph, “a sequence of five letters used to represent a single sound (phoneme), or a combination of sounds, that do not correspond to the individual values of the letters.”
The best-known example is the German tzsch, which stands the simple sound /tʃ/ (represented in English by the digraph ch), as in Nietzsche. The usual German way of representing this sound is the tetragraph tsch, itself a combination of t and the trigraph sch representing /ʃ/ (in English the digraph sh).
In French, the pentagraph eaulx is one of the many ways of representing the simple sound /o/ a the end of proper names, as in Meaulx, in addition to the tetragraphs eaux and ault (as in Renault) and numerous trigraphs (aux, aud, aut) and digraphs (os, ot, od); there are probably others. Sometimes one even finds a simple o, as in the case of the actor Jean Reno. But that would be too simple for French; Jean Reno is originally a Moroccan-born Spaniard named Juan Moreno. (Of course, there are many French people with Italian or Spanish surnames ending in o, like the former director of the Metropolitan Museum, Philippe de Montebello, or the current mayor of Paris, Anne Hidalgo.)