Reference lines are horizontal or vertical lines that span the data region of a graph to designate goals or demarcations.
You can add reference lines to most Minitab graphs.
You must use the scale units on the graph to position the reference lines. For example, if the y-scale has been transformed so that the tick marks are displayed in transformed units, you must enter the reference line position in the transformed units.
Reference lines are displayed behind data displays and may be difficult to see with some graphs, such as histograms. To create a reference line that sits in front of the graph's data display, use a calculated line instead. Enter coordinates in your worksheet representing the two end points of the reference line, then use to draw the line.