Minimizing Cognitive Strain

William Cleveland wrote the classic "The Elements of Graphing Data" which has been informing data visualization efforts for over 20 years now. He is a big proponent of exploratory data visualization. But perhaps the most essential point he makes is that:

Visualizations should maximize information content and minimize cognitive strain.

Remember that your audience is busy. They probably don't have time to laboriously interpret your work. Your audience also is probably not as familiar with your data as you are, nor can they read your mind. To be effective, there needs to be enough information to tell a story and the story has to be obvious. Make it easy. Spoon feed your audience, not because they're dumb, but because they only have a few moments to spare before moving on. This is your chance to educate, inform, perhaps even surprise and captivate. That will only happen if the story from the data is glaringly obvious.

A simple example from the Win-Vector blog (a great resource for data visualization and data science):

 
These two plots contain the same information (number of households per state), but in the first case, the states are sorted alphabetically, and in the second case, by the number of households. The simple act of sorting made it easier for anyone to recognize that Wyoming has the smallest number of households and that California has the most. It's also easier to get a feel for the distribution this way.

Of course, the more data we layer on, the harder it is to interpret. Always remember William Cleveland: Maximize Information but Minimize Cognitive Strain.