Do I need a legend?

Legends explain graphics; at least, this is what we hope for. Humans are spatial animals very much dependent upon perceptual information. Keeping this in mind, graphics should use both perceptual and spatial relationships to make efficient use of our traits. Unfortunately, graphs that use arbitrary symbols require substantial cognitive overhead that takes away mental power we would otherwise use on understanding the main facts in the graph. This diversion of “brain power” may lead to errors and misinterpretations. To keep the naturalness principle* intact, ask yourself do I need a legend or can I design my graph without a legend and its arbitrary symbols, colors or shades? If you decide a legend is necessary, make the representation as meaningful as possible.

*More on naturalness can be found in The Design of Everyday Things by Donald A. Norman. Cambridge, MA: Perseus Books, 1990