We develop small to large visualization projects for different tasks and industries and sometimes while rewriting them a couple of times in the process we hit walls because we discover that we need to add a lot of code to support new requirements. Now we have established a design process that seems to work well (at least we reduced the development time for each new project quite a bit), but we're still left scratching our heads around this question: what exactly should we test when testing visualizations?

• If everything that we want to explore is on the screen (bounded visualizations)?
• If the data is ok - if data is valid (that's one of the nice things about visualizations you can spot errors in your datasets)?
• Usability?
• User interaction?
• Code quality?

I can tell you for sure that a simple check of the code quality is certainly not enough! Is there a classic paper / book about how to test visualizations? Also do you happen to know about classic design patterns for visualizations (except the obvious ones like Pub-Sub)?