Select the chart
Click the data series you want to add a trendline to
Right-click the selected data series
Click Add Trendline
Choose the trendline type
Format the trendline if needed
Close the pane or dialog when finished
Select the chart
Click the data series you want to add a trendline to
Right-click the selected data series
Click Add Trendline
Choose the trendline type
Format the trendline if needed
Close the pane or dialog when finished