To add a constant value to a plot you will have to define a new object of the class VisXvalue inside of the plot object and to fill all needed parameters. For example via:

Xline = plot.CreateObject('VisXvalue', 'Constant');

One very important parameter is hidden and it is the one containing the information whether the constant is a X or Y constant.

Xline:cConst = 'Y';   !to set Y line write 'Y'; to set X line write 'X' i.e. it is a string parameter

See attached pfd file