I'm not sure if this is really a fully reliable workaround but if it's a one off I used a level of detail calcuation to work out the average. I then added this as a secondary measure, synchronised the axis and turned the transparency up to 100%.
After that I used a YTD growth table calculation to add the label.
Please find workbook attached, and hope this helps
perc_dif_years_ref_line ps.twbx 29.1 KB
Great ideas here, thank you for taking the time. it doesn't appear we need the synchronized axes. I just created a calculated field where everything except December values is null. I then used a Table calculation (% difference from previous year) as the label. It's still not in the center between the lines, but may do.
The workbook is attached.
perc_dif_years_ref_line.twbx 21.2 KB