Show trend lines

Select a 2-D column, 2-D Line, Bubble, Scatter, or Combo chart.

Double click the chart to open the Chart pane, and select Format.
If you don't see Format, select an element in the chart. 

Expand a Series for your data and select Trendline  .

Select a 2-D column, 2-D Line, Bubble, Scatter, or Combo chart.

On the Chart Design tab, select  Add Chart Element and choose  Trendline.