In my first attempt, I made a date (FullDate) out of the Year and Month. Using Month(FullDate) and changing it to continuous, the graph can be produced simply by accounting for nulls with ZN([Sales]). It looks like the values at the end are not added.
After looking at your solution and the posted workbook, I learned that lines could be drawn just by changing the mark type!
I came up with a different expression: