Hello Matthew Forrest,
You do not mention which version of Tableau you are using so hopefully my example workbook can help you out as it is in V10.
I have created a calculation [Calc to Show Ref or no] and set it to:
If Window_Min(Min([Order Date]))>#1/1/2011# then #7/4/2015# else null end
This calculation is then set for the value of the Reference line in the view. If there is a value in the view that is less than 1/1/2011, the ref line disappear. However, if a value exists that is higher, the ref line appears. You can do the same sort of thing (and even use another parameter to potentially "show" or hide all lines if you like. Anyway this should get you going.
This also seems like the sort of thing Rody Zakovich would be all over so I am including him.
Here are the two views:
With no data lower than 1/1/2011 - Ref line for July 4 of 2015 appears.
Here we have a date going all the way back to Feb of 2010 - and the Ref line is gone and the months have collapsed.
disappearing ref line.twbx 1.9 MB