Add line separator between years in chart

I don't know if this is even possible but I am trying to come up with a way to add a line separator (or something similar) between years for a chart that is interactive (I have a scroll bar that is linked to the chart and 10 years of monthly data).

The axis labels are formatted as "mmm" and I don't want to change to format "mmm-yy". I am hoping there is a way to add this separator as it is very hard to identify what year a user is looking at as he/she scrolls through the data.

I have attached a sample of spreadsheet that better clarifies how it is currently set up and the desired look.

If this is not possible, any other suggestions are surely welcomed. Thanks

Add another series as xy-scatter on secondary axis.
Use Y error bar to draw the line.

I have added code to update the scrollbar and secondary horizontal axis when the number of points changes, via Change event on E4.

There are also 2 new named ranges.