You can add trend lines to any of the chart widgets on your analytics dashboard using the Analyse it tool.
To do this, simply hover over the chart widget on your Analytics dashboard to reveal the Analyse it option.
To switch a the Trend line on, simply click on the toggle to activate:
You will now see a trend line on your chart:
The Trend Information option will show you details about the trend of your chart, including the type of Trend line being used, trend averages, etc.:
You can specify the type of Trend line you wish to use, along with other options using Trend Settings: