From his book “The elements of graphing data”
“When a graph is constructed, quantitative and categorical information is encoded, chiefly through position, size, symbols and color. Whean a person looks at a graph, this information is visually decoded by the person’s visual system. A graphical method is successful only if the decoding process is effective. No matter how clever and how technologically impressive the enconding, it is a failure if the decoding process is a failure”.
William S. Cleveland
(click to enlarge)