The following is a quotation from Edward Tufte's book, The Visual Display of Quantitative Information:
"Most principles of design should be greeted with some skepticism... we may come to see only through the lenses of word authority rather than with our own eyes."
While establishing the purpose of the visualization project sets the desired tone of the design and its function, there are inevitably many other factors that will have a significant influence on the shape and direction of our visualization design.
It is especially important to identify and recognize the impact of the contextual conditions, within and around your project that will affect what you can and can't achieve and how you might achieve it.
This list of factors may seem quite obvious and fairly rudimentary, but if we wish to eradicate the likelihood of misjudgments or misunderstandings, and maximize the efficiency and effectiveness of the process, we need to nail them early on...