Vertical Line on a 2Y axes chart

[sahafi]

How can I have a vertical line extended all the way in a line chart that have
2 Y axes?
I have been using the horizontal/vertical line techniques without any issue
until now, when my current chart carry values in KGs in the primary Y axis,
and the same values in Lbs in the secondary axis. If I check out the
secondary axis, the line will extend all the way, but I will lose my Lbs
values. If I leave it checked, the line will cut short half way! Any trick to
by pass this?

Thanks.