No, you do not need to, and will not generally be able to, describe everything that a graph conveys in text. Graphs can give you an intuitive understanding of the data that text would not be able to, simply by virtue of using other parts of the brain and requiring less short term memory. If a graph can be replaced with 5 pages of text, that doesn't mean that you get the same information from both - you're likely much more able to keep one image in your short term memory than 5 pages of text.