How do I show reference lines for a specific trellis page/panel?

I have a line chart that is trellised by column name.  I would like to add a reference line to each graph--the value of the reference line depends upon the column/trellis panel.  Right now, I can add a reference line, but it shows up on all of the graphs.  Is there a way to specify which reference lines show up on which graph/panel?



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