Use the Reference Lines dialog box to specify reference lines for a graph. For example, in a graph that plots scores of students' projects, you might use a reference line to show the minimum score for a passing grade.
You can display three reference lines for each data axis in a graph.
Each reference line has the following options:
Show box -- Specifies whether the reference line appears in the graph.
Color box -- Specifies the line color of the reference line.
Thickness box -- Specifies the thickness of the reference line.
Label box -- Specifies the label for the reference line. Click in the box to type a label. The label appears in the legend of the graph.
Cross at box -- Specifies the value, along the axis, at which the reference line appears.