Here is one way...
First, as you are sharing an axis I used measure names/measure values. I also changed the filter to show last 2 years
Next I created the following Table Calculation to get last years sales
[Actual Sales - LY]
I can then bring this in to the chart, so we get this
Finally we can 'Hide' the 2018 (we can't filter as the Table Calculation needs access to the 2018 data)
Hope that does the trick and makes sense.
Thank you for the quick solution!