Show a Movable Line on a Graph

You can add an unfitted, movable line to any graph that has numeric attributes on both the horizontal and vertical axes (both the X and Y axes).

1. Select the graph.
2. Click the ruler icon in the right corner of the Graph window.
3. Check Movable Line, as shown below. You can then click and drag the line so that it goes through the points on your graph. Note that the equation used to produce the line also appears on the graph in a yellow box (you can drag the box to a new position in the graph if you want to). The equation will change as you drag the line.
   
4. If you decide that your line should always pass through the origin (0, 0) on the graph, click on the ruler icon in the inspector panel to the right of the graph, and choose Intercept Locked from the menu, as shown here: