A graph or chart can give the reader a visual representation of a great deal of data. Concepts or results can be more easily grasped by a well formatted graphic.
Charts, usually, take up more space in a document than is absolutely required.
Edward Tufte has come up with the concept of Sparklines (Sparklines:Intense, Word-sized Graphics)
These are small graphs about the same height and width as common words. They are not out of place in the text of a document.
Sparklines give the reader a snapshot of the data that quickly supports the material being discussed.