I'm not familiar with any way to make a recurring reference line - perhaps someone else knows a way to do that - but as long as the display set is of known and limited length, you could set up multiple reference lines (e.g. 7 if you're working with weeks), using the formulas:
floor(Max(myDate))
floor(Max(myDate))-1
floor(Max(myDate))-2
etc.
This will also work for any shorter set, since the reference line will just be off-graph, but it won't work if you're potentially working with very long time displays, since you don't want to set up that many reference lines.
