Select the series for which you want to show a trendline, and then click the Inspector button in the toolbar.

Click the Chart inspector button, and then click Series.

Click Advanced at the bottom of the Series pane. Then click Trendlines.

Select, from among the following, the type of equation you want to use to calculate the trendline:

Linear:  Produces a best-fit straight line, most useful for simple, linear data series.

Logarithmic:  Produces a best-fit curved line, most useful when values increase or decrease quickly, then level out.

Polynomial:  Produces a curved line that has hills and valleys to show where values rise and fall. Specify the order of polynomial you want to use in the Order field; order 4 produces as many as 3 hills or valleys.

Power:  Produces a curved line for use when you want to compare measurements that increase at a specific rate. Power can’t be used with data that contains zero or negative values.

Exponential:  Produces a curved line for use when values rise or fall at increasingly higher rates. Exponential can’t be used with data that contains zero or negative values.

Moving Average:  Produces a trendline using the number of data points you specify in the Period field, averages them, and uses the average value as a trendline point. Specify the period you want to use in the adjacent field.

To see the equation that describes a trendline, select the line, and then select Show Equation. You can drag this equation to wherever you want it to appear on the slide.

To see the R-squared value that was used to calculate a trendline, select the line, and then select Show R^2 Value. You can drag this number to wherever you want it to appear on the slide.

To display a label for a trendline, select it, and then select Label. Type a label in the adjacent field.

To change the line color or thickness, select it, and then click the Graphic button in the Inspector window. Make adjustments using the Stroke controls of the Graphic inspector.