The Scatterplot Options: Fit Line dialog box allows you to choose different types of 
regression lines and how they fit your data.  To open the scatterplot options dialog box, 
type Alt+C and O while the chart you created is open.  In the dialog box, use the Tab and 
cursor keys to navigate.  When you have selected the type of line you want, type Tab until 
the "Define" button is selected, then type Enter.