1. Click the gear icon in the upper-right corner of the Graph window to see the commands in the Graph Gear drop-down menu.
2. Choose Show Movable Line, as shown below. You can then click and drag the line so that it goes through the points on your graph.
3. If you decide that your line should always pass through the origin (0, 0) on the graph, choose Lock Intercept at Zero from the Graph Gear menu, as shown here:
Note: To remove a movable line, choose Hide Movable Line from the contextual Graph Gear menu that now appears.