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Short Desciption: Graphic Examples Graphic 1 Graphic This document provides examples of a number of graphs that might be used in understanding or presenting data. Comments with each example are intended to help you understand why the data were plotted i
Content Inside: Graphic Examples Graphic 1 Graphic This document provides examples of a number of graphs that might be used in understanding or presenting data. Comments with each example are intended to help you understand why the data were plotted in a certain fashion, or why it should have been done differently. O PLOT OR NOT TO PLOT The purpose of plotting scientific data is to visualize variation or show relationships between variables, but not all data sets require a plot. If there are only one or two points, it is easy to examine the numbers directly, and little or nothing is gained by putting them on a graph. Similarly, if there is no variation in the data, it is easy enough to see or state the fact without using a graph of any sort. When a graph is appropriate, it must be of an appropriate type to avoid misleading the reader. Fig. 1. Both plots in figure 1 show US research expenditures by discipline in 2000. The scatter plot on the left is incorrect because it implies a relationship between the variables on the two axes, further reinforced by the connecting lines. Since the horizontal axis is just a list of disciplines with no inherent ordering, no relationship can exist. Categorical data of this sort are better plotted as a bar graph, as on the right, since such a graph displays the relative magnitudes without implying a functional relationship. (Pie charts are often seen in the popular press for financial data, in order to emphasize the relative size of the allocations. Pie charts are rarely used in technical fields.) SET OF COMMON MISTAKES It has been