Q:

Should [charts] and [graphs] tags be merged?

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?

A:

Per Wikipedia terminology:

chart: A chart is a graphical representation of data, in which "the data is represented by symbols, such as bars in a bar chart, lines in a line chart, or slices in a pie chart".
graph: Information graphics or infographics are graphic visual representations of information, data or knowledge intended to present complex information quickly and clearly.

Thus, I propose to merge (and synonymize) into chart.
During cleanup, I would propose to replace graph with chart where appropriate. In cases there graph is used to refer to non-chart type graphs, use a different tag or just remove it.

