If you need to get the Y position of a vertically scrolling container, create a text box. Then on the container scroll trigger, set a variable that evaluates the numerical callback value from the event (Y position). Then add a callback that changes the text property of the text box by reading from the variable.
Re-creation of the onboarding sequence of the app "Service"
See how it was done step-by-step in the on-boarding webinar:
http://docs.proto.io/videos/creating-great-on-boarding-experiences-with-proto-io/
The app itself:
https://itunes.apple.com/us/app/service/id1005945537?mt=8