In the Graph Options dialog, under Axis, select Reference Lines to access the options described here. Reference lines can be added to any graph. A reference line is a line on the graph that is parallel to the associated axis. Reference lines can be used to mark specific X or Y (or in 3D graphs, Z) values. A reference line is associated with an axis, and each axis can have 1 or more references. You can adjust the color, label, and other aspects of the reference line.
Axis. From this drop-down list, select the axis for which the reference line will be used.
Line. From this drop-down list, select a specific reference line from among all the previously created reference lines for the specified axis. Once selected, the properties can be changed or the reference line can be deleted.
Add new line. Click this button to add a new reference line to a graph for the selected axis.
Delete. Click this button to delete the currently selected reference line from the graph.
Text box. A reference line can have optional text that is displayed as an axis value label; enter this text here. The text can be formatted using the options above the text box: Font, Font Size, Text Color, Bold, Italic, Underline, or Subscript.
On. Select this check box to display the reference line. When this check box is cleared, the reference line will not be displayed on the graph, but the specifications will remain so that it can be turned back on at a later time.
Show value. Select this check box to display the numeric value of the reference line instead of the label (even if the label is blank). (See also the Value and the Value format options described below.)
Show tickmark. Select this check box to display a tickmark at the start of the reference line.
Value. Enter a numeric value (or use the microscrolls) for the reference line, defined by the associated axis.
Line pattern. Click this button to display the Line Properties dialog, where you can define the line pattern/color for the reference line.
Tickmark format. The options under Tickmark format are enabled when the Show tickmark check box (see above) is selected. Specify here the display attributes for the tickmark. In the first drop-down list under Tickmark format, select whether you want the tickmark to be Inside the graph axis, Outside the graph axis, or Crossed (both inside and outside).
Size. Select the size of the tickmark: Small, Medium, Large, or Custom. When Custom is selected, use the Points option (described next) to specify the size.
Points. Enter a numeric value (or use the microscrolls) for the line weight of the tickmark.
Value format. When the Show value check box is selected (see above), select here the numeric display format for the tickmark value.