One of the many insights from Don Norman's great design book is that a user's behavior is affected by "knowledge in the world", and "knowledge in the head." Applied to graphics, this means readers of graphics use both knowledge in the chart and knowledge in the head. The recent debate between Alberto Cairo (and various others) and Andy Kirk about the following map illustrates this well.
As background, we should note that Cairo is Spanish-born and has taught at the Universities of Miami and of North Carolina while Kirk is a UK-based consultant.
Cairo is dismissive of this map, because the blue-orange split is really caused by the underlying population distribution. Notice that in coming to this conclusion, Cairo is using both knowledge imparted by the map and knowledge that pre-exists in his head. Someone who doesn't know the population density of the U.S. would not readily see the underlying cause of the pattern.
For those who can tell that the orange areas are the major urban areas, we are still making an assumption that every important metro area is painted in orange, and that there are no non-urban area painted in orange. Do we really know the validity of these assumptions?
It is understandable that UK-based Kirk brings less knowledge of U.S. geography to the table:
Whilst I know roughly where the major cities of the US are, the size and population-density extremes of the country fascinate me so I find it interesting, particularly as a non-US person.
Graphics designers ought to think about how much knowledge the readers would be required to bring with them. It seems inappropriate to be imparting demographic lessons from a chart that depicts economic activity. Is there a way to provide Kirk with the lessons he desires without boring Cairo with "common knowledge"? That is the challenge here.
Please read the post on my sister blog for commentary on other aspects of this map. See this link.