"A picture is worth 1,000 words." Data visualization is based on this old adage, pointing out the value of displaying quantitative information as images or graphs. The idea is to convey the meaning of the data in a simple and intuitive manner. This is achieved by mapping quantity into geometric attributes, length, area, color, symbols, positions, curves, or other visual cues. Common graphs are histograms or bar charts, displaying quantity as length; pie charts for presenting parts of a whole; time series, which show the development of a variable over time; and scatterplots showing the relationships between two variables by marking pairing quantities. These graphs are abstract, but use real-world properties to support intuitive understanding. A pie diagram makes associations to bakery goods, time series to mountains and valleys, and scatterplots may be discussed using terms such as position, closeness, distance, groups, and outliers or...