From version 7.5 it is possible to add trendlines to Small Multiple Charts, these can either be added to an individual series by right-clicking on it and selecting "Add Trendline" or the entire chart by right-click that.
Select the series for the trendline
Provide a title for the trendline to be displayed on the legend
Select a colour of the trendling
There are various options for calculating the trendling, select the appropriate option.
Editing and Removing
Right-click on the trendline legend to edit or delete it