This visualization of gas prices by county in the United States is timely as high gas prices are not only impacting pocketbooks, but are also becoming political taking points for presidential candidates and this issue may drive policy. This shows the regional variations in prices (so sorry to my California friends), but it is a great launching point for asking the questions: why are the prices for a certain commodity higher in one region than another? What factors lead to the spatial differences in the relative economic value in one region over another? Supply and demand works beautifully on a two-axis graph, but supply and demand happen somewhere, giving a simple chart added complexity since it's spatially contingent and we must make the assumption and caveat explicit.
Via Seth Dixon