Noah Iliinksy spoke at UW, yesterday, on the topic of Effective Visualization. Iliinksy has a new book out, Designing Data Visualization (review), and served as editor of Beautiful Visualization, both from O'Reilly. And, yay Seattle, he lives here in town and has a degree from UW.
If you had to sum up the talk in a sentence, it would be this: Take the advice from your college technical writing class and apply it to data visualization. Know your audience. Have a goal. Consider the needs, interests and prior knowledge of your readers / viewers. Figure out what do you want them to take away. Ask, “who is my audience, and what do they need?” I guess that's more than a sentence.
The human eye is great at perceiving small differences in position. Use position for your most salient features.
Color is often used poorly. Question: Is orange higher or lower than purple? Answer: No! Color is not ordered. However, brightness and saturation are and can be used effectively to convey quantitative information. Temperature is something of an exception, since it is widely understood that blue is cold and red is hot. Also, color is often loaded with cultural meanings - think of black hats and white hats or the political meanings of red, orange or green, boy/girl = blue/pink, etc.
Appropriate encodings by data type
Click to expand this handy chart!
As an example of how to do it right, Iliinsky points to Hipmunk, which crams an enormous amount of data into a simple chart of flights from Seattle to Phuket, Thailand.
We can see departure and arrival time and duration, in both the absolute and relative senses, plus layovers, airline, airport and price. And, you can sort by "Agony", which is cool. They've encoded lengths (of time) as lengths, used text (sparingly) for exact amounts, color to show categorical variables (airline) and iconography to indicate the presence or absence of wireless internet on flights.
The cool chart and the quote about encoding, were expropriated from the slides from Iliinksy's talk at Strata. If you want more, there's a video of a related talk on You-Tube and a podcast on Letting Data Tell the Story. Tools recomended by Iliinsky include R and GGPlot, D3 and Protovis, and Tableau.