Vertical line through log plot

I'm trying to present a dynamic constant on a log that is not related to depth. For example, I want to show calculated Ic from a CPT on a Plot vs Depth log and overlaying the cohesive/non-cohesive cut off as a vertical line, I need this line to run from the very top of the log to the very bottom.

If I use the CPT depth, the cut off line only draws to the depth of the CPT and not the page.

I need the vertical line to by dynamic and changeable by the user so unable to draw it directly on the plot. 

Is there a way to enter multiple depth values into the depth expression field instead of referencing a field in the database?