How to add grid lines at specific position to a plot?

Hi all,

[Disclaimer: I have done extensive and intensive searching on Google and
asked around but couldn't find a solution to this problem.
Please help me instead of just pointing out how simple and stupid my
question is. God bless good folks! Thanks!]

This is a headache for me and I have been struggling with this for a while.

With an existing plot, how can I add a horizontal grid line at vertical
location 0 (just need 1 grid line)...

I can certainly draw such a line myself using "lines".

However, that will impact my legends because that also counts as a line.
But we wouldn't want a grid line to be counted as a line...

Could anybody please help me?

Thanks!

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