The Overlay drop zone in Graph Builder is my favorites -- in my mind, it means "make a new line with a new color for each different value of this variable".
I would love to see it broken into two boxes -- "Overlay-Color" and "Overlay-Style". GB would change the color for the variable dropped onto Overlay-Color, and the line style for the variable dropped onto Overlay-Style. This would allow the user to easily create solid, dashed, etc., lines in different colors.