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Double-click the Excel workbook document in which your data is stored. If you don't have the data that you want to analyze in your spreadsheet, you'll instead open Excel to create a new workbook. You can then enter your data and create a graph from it. Click the series of data that you want to analyze to select it. If you haven't yet created a graph from your data, create one before continuing. It's at the top of the Excel window. This option is on the far-left side of the Chart Design toolbar. Clicking it prompts a drop-down menu. It's at the bottom of the drop-down menu. A pop-out menu will appear. Depending on your preferences, click one of the following options in the pop-out menu:  Linear Exponential Linear Forecast Moving Average You can also click More Trendline Options to bring up a window with advanced options (e.g., trendline name). Press ⌘ Command+Save, or click File and then click Save. If you've never saved this document before, you'll be prompted to select a save location and file name.
Open your Excel workbook. Select the data in your graph. Click the Chart Design tab. Click Add Chart Element. Select Trendline. Select a trendline option. Save your changes.