Here's the solution fyi:
Right click on the graph then choose Customize
The way this works is a bit non-intuitive, but you have to move Bar to the top of the list using the arrow keys. That puts Reference lines on top of the data where it's visible. The default was Reference Lines on top, which I thought was the correct position to make them visible. AFAIK there's no option in Axis
Settings to override this default.