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Stephen Few, a respected IT innovator, consultant, and educator has studied the art and science of visual presentation for many years. In a White Paper prepared for Cognos Corporation, he presents seven principles for the effective display of quantitative information. The fourth of these is presented below:
“Graphs are sometimes intentionally designed to deceive, to misrepresent the truth by visually encoding values in a way that does not correspond to the actual values themselves and the differences between them. Even more often, however, people unintentionally misrepresent data in this manner, simply because they don't understand this principle and how to follow it. The most common way that this occurs involves bar graphs with quantitative scales that don't begin at zero. Because the lengths of bars encode the values they represent, the full length of the bar must be displayed, beginning from zero, for the values to be encoded properly." Few refers to a graph and says "notice that actual sales in the East region appear to be twice as great as planned sales, but in fact, this is far from the truth. Actual sales are only 5 percent greater than the plan. When you use a graph to communicate, people should be able to look at the graphical representation alone to compare differences in values. If the graph doesn't support this operation, what's the point of using a graph?”