Found on Interaction Design Foundation:

Free chapter from The Encyclopedia of Human-Computer Interaction, 2nd Edition

Chapter 35: Data Visualization for Human Perception

To visualize data effectively, we must follow design principles that are derived from an understanding of human perception.

As the saying goes, "a picture is worth a thousand words" - often more - but only when the story is best told graphically rather than verbally and the picture is well designed. You could stare at a table of numbers all day and never see what would be immediately obvious when looking at a good picture of those same numbers.