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When you create a scattergram, the most common additional step of analysis and visualization is to superimpose on it a trend-line. If you use the default trend-line function of Excel, the line will be straight (known as the least squares line), and it will generate an equation for the line. A straight line, however, may convey a distorted sense of the trend of the points on a scattergram that has some unusually high or low numbers or anomalies in the middle of fairly consistent numbers. True, you can selectively delete the outliers, but doing so leaves you vulnerable to charges of data manipulation.