Editor > Add > Reference Lines
Choose to display reference lines. By default, Minitab displays labels for each reference line drawn. Customize reference lines using scale editing.
Show reference lines at Z values: (3D graphs only) Type the z-scale values for displaying reference lines.
Show reference lines at Y values: Type the y-scale values for displaying reference lines.
Show reference lines at X values: Type the x-scale values for displaying reference lines.
Show reference lines at data values: (Categorical graphs only) Type the data values for displaying reference lines.
Show reference lines at category scale positions: (Categorical graphs only) Type category-scale values for displaying reference lines. Values that do not correspond with a categorical value will not be labeled.
Show reference lines at time scale positions: (Time series plot and area graph only) Type time-scale values for displaying reference lines. Values that are not integers will not be labeled.