Are combination charts still available?
Yes, combination charts are still available in Version 3. V3.2 came out yesterday so if you need an upgrade, get in touch with whoever supplied you the system and ask.
It depends where you are building your chart overlay. If you want it on the Measure or in a dashboard, then go into edit and click on the small cog on the top right of the chart and select Edit Chart. You can then hit the Add Series button and select the overlay.
If you are in the new Charts and Reports section then go into edit and immediately hit the Add Series button.