Here is how I did what I think you want
1. Drag the x (site_order) and y(daily.survival.rate) columns into the graph.
2. Drag the Treatment column to the overlay
3. Click on the graph and select
Add==>Line
4. Two lines will appear. One for each Overlay. So what you need to do, is to go to the Properties Area on the left of the graph, and find the Properties for the Line. Go to the Variables area and click on the gray triangle to open up the variables list. Unclick the Overlay for the Treatment variable.
There will then only be one line.
I believe this does what you want?
Jim