I've just found a good example to illustrate my point. Ask yourself if this graphic lets you do anything with the data —comparing values, seeing relationships between them, etc.,— or if figures have been arranged to create nice-looking picture, instead:
(UPDATE: Stefanie Posavec and Moritz Stefaner have suggested the term “data illustration” rather than “decoration”, as it sounds less demeaning. I disagree. I love Baroque architecture, so I think that decorative art can be valuable, and can be done well or badly —perhaps the case here. But I am fine with “data illustration”, too.)