Hi @hillel ,
One possible option is to move one of the Y variables to the right side of the graph and then select the line graph style in Graph Builder (the left Y variable will be the line). I was able to get this to work with the Big Class data set, graphing weight vs age and height vs age -- weight as the fill to bottom area under curve and then height as a red line, I even wrapped it by "sex", so it's something like your data. I changed the axes so that it kind of separated the weight/height data, so it's easier to see.
I've attached a screen shot of what Graph builder looks like.
This is one possible solution, but I'm sure there are others, and someone else is sure to have a better solution.
Hope this helps!,
DS