There are two ways of adding trend/best fit line - Statistics and Regression analyses. First one is designed for typical Time charts, second for Scatter charts.
Line from Statistics
In the Statistics analysis, add a “Trend” on the series of interest. If needed, set the data range on which the trend line should be calculated, and untick “Extrapolate” if you want the trend line only to be drawn on the defined range of calculation.
This will yield a trend line calculated using linear regression.
Line from Regression
This is the only way to add best fit line in a Scatter chart.
- In the Regression analysis, make sure you have selected to output the dependent and the explanatory series
- Add a Scatter chart from the Regression analysis
- Go the Scatter chart and open the Graph layout menu
- In graph layout, you need to set two pairs of lines
a) Pair #1: the explanatory and the dependent series
b) Pair #2: the explanatory and the predicted series
Example file: Regression line
For more information, check: