Next, right-click any one of the blue data points, and from the resulting pop-up menu, select Add Trendline (the blue dotted line in Image 3). This resulting blue trendline represents the one true linear line that dissects the data points in such a manner that when all the distances from each data point to the trend line are squared and added together, they produce the least amount (hence, the least-squares method). To depict the process further, I have extended the linear line and added data points at intervals equivalent to the actual data intervals (as illustrated by the red dotted line, data points, and interval arrows in Image 3). The red data points correspond to the future values calculated in cells F7 through H7 in Image 2. Image 3: Excel scatter chart and trendline This scatter chart and trendline visually suggest how Excel's Fill Handle tool works - and how the actual historical data are used to project the future data points. To prove the Fill Handle's accuracy, these same projected numbers can also be calculated mathematically using Excel's SLOPE and INTERCEPT functions, as follows.
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