charts - 205 questions tagged / 77% answered.
graphs - 148 questions tagged / 79% answered.
Both charts & graphs - 27 tagged with both / 74% answered
I think the real difference between charts and graphs is so technical and nuanced that there is no practical difference in how most users (myself included) view them.
EDIT: Here's a current example where the question is tagged charts but references "graphs" throughout the question.
Should they be merged or made synonyms?