Specify a Math Expression to Define the Datum Line Display
 From the modeling window or the Plot Browser > Datum Lines view, select the datum line to edit.

From the Entity Editor or the Plot Browser > Datum Lines
view, in the Position field, enter the following:
 For a horizontal datum line, enter a math expression based on “x”, f(x) and press Enter.
 For a vertical datum line, enter a math expression based on “y”, f(y) and press Enter.
The datum line display is updated to reflect the expression.