Here's why:
* Root words are the basic building blocks of words. They carry the core meaning of the word.
* Prefixes are added to the beginning of a word.
* Suffixes are added to the end of a word.
"Graph" appears in many words like:
* Autograph (self-written)
* Biography (life written)
* Graphic (relating to drawing or visual representation)
* Graph (a visual representation of data)
In all these cases, "graph" is the core element that refers to writing or visual representation.