Learn datavis before learning the tools

Datavis is the art and science of turning data into graphics, using a visual language to help a user extract useful information from his data. A lot of good books will describe it better than I do. But another definition I would bring is: datavis is what a datavis designer does. I will try to describe datavis from its practice. Basically, starting from my own experience, I want to explain the part they don't tell us at school.