What are the goals of visualization? And what are the features that support those goals?
My 10 cents worth:
The basic goal is to facilitate reasoning and thought about what is being visualized. That reasoning could revolve around causality, hypothesis, predictions, inferences, habits, modus operandi, contradictions, uncertainty, and a whole host of problems the user is trying to solve. Often the reasoning revolves around external data and/or knowledge too. Visualization should expose structure in the data such as patterns, clusters, gaps, bursts of activity, outliers & trends, etc. And at the end of the reasoning process the great thing about visualization is that one should end up with a picture that can be used to disseminate one’s insights to other people.
So what key features enable these goals to be achieved?
* A Summarization/Overview to give the big picture
* Zoomability
* Drill-down on data for the detail
* Easy navigation around the visual
* Filtering information by category or query
* Different types of visualization expose different patterns (geographic, timeline, textual, lists, link diagrams, etc.)
* Brushing & linking visualizations together can help the filtering & exploration
Other basic things which must be in place in order to succeed:
* Ease of import and export – and adhering to any standards
* Some basic searching of the data
* One must be able to read the data – in particular any text
* Scaling well as the data size gets very large
* Links out to other systems for further information is key
* Links back in to the visualization from other systems can also be powerful
* Interoperability with other visualization tools and other applications in general
* Commentary, scribbling and drawing on the visualizations is a great way to add understanding – a picture alone is rarely enough
And don’t forget the more esoteric things too:
* It needs a positive emotional response so it must look good and not conflict with user’s expectations
* It can use standard visual symbolism, conventions & metaphors
* It must use the basic visual variables well (shape, colour, position, etc.)
* Transitions between visualizations must be smooth to allow the user to keep their context
* It should use design techniques like ‘information scent’ & obvious affordances
* It should facilitate playfulness where ever possible – don’t punish ‘mistakes’!
Phew – glad I got that off my chest – back to the day job 
This looks like an interesting blog. I ran across a shortcoming, however. When I click on a label to see related posts, the resulting page has the content of the posts, but no links to the individual posts.
“Different types of visualization expose different patterns (geographic, timeline, textual, lists, link diagrams, etc.)”
yes, and mix of all of these : flowing transition from one to the other, mixing two (time + geo, etc.)
something like “data agility” ?