Activity of counties in the space of 12am and 1pm. Each dot represents a county, and the horizontal axis measures the relative tweeting activity between 12am and 1am in that county, while the vertical axis represents the relative tweeting activity between 1pm and 2pm in that county. As these two measures are correlated, a linear transformation could combine them into a single coordinate. The new coordinate axis is represented by the dashed line. The black arrow points to the average of the measures along the original axes. The blue and the red arrows are possible choices for A and B vectors, see Section 2.4.