Trend lines have become popular questions in recent Tableau examinations. Follow along:
1) First drag Sales to the Rows shelf and Profit to the Columns shelf:

You will only see a single mark since the view is aggregated.
2) Now, break down this view by state. Drag State into Detail on the Marks shelf ( or directly to the view):

3) Finally, move to the Analytics pane, and drag Trend line to the view. When you drag it, select the Linear option!:


4) The following is our view. Hover over the trend line to see the R-squared value: