99% of the time it is design, 1% it is art. The goal is always to convey information in a digestible way without being misleading. An example of it being art is that video that illustrates death counts in WWII.

The Wall Street Journal Guide to Information Graphics should be a required for reading for anyone working with numbers.

tldr: only use color when necessary, avoid bold typefaces, keep scales consistent across charts unless there is a specific need to do otherwise.