Reference lines for Zone Chart

Stat > Control Charts > Variables Charts for Subgroups > Zone > Scale > Reference lines

Display reference lines at one or more values on the x-axis or the y-axis. Separate multiple values with spaces.

Show reference lines at Y values
Enter the y-scale values to display reference lines at.
Show reference lines at time scale positions
Enter the x-axis values to display reference lines at.

The following example shows a vertical reference line at 30 on the x-axis and a horizontal reference line at 0.65 on the y-axis.