In Excel, once you have a chart with data in it, click on the green “+” icon in the upper right-hand corner of the chart and check off the Trendline box. To use different types of trendlines: You will notice that there is a sideways arrow next to the Trendline box where you can choose different types of trendlines: exponential, linear forecast, or two period moving average. For a linear trendline: Select two cells next to each other in the same row. For a scatter chart, enter the formula. =LINEST(y-range,x-range) where y-range is the range specifying the y-values, and x-range is the range specifying the x-values. Confirm the formula with Ctrl+Shift+Enter to make it an array formula.