I thought I explained the requirements but let me know what part is not clear in my original post.
If I select "2019", "December", I expect the chart to display 6 months (ungrouped) including Dec: Jul, Aug, Sep, Oct, Nov and Dec of 2019 (FY, ie. 2018 calendar year in Australia)
If I select "2018", "December", I expect the chart to display 6 months (ungrouped): Jul, Aug, Sep, Oct, Nov, Dec of 2018 (FY, ie. 2017 calendar year in Australia).
No hard-coding years and months in measures would work - I can choose any year and month in the slicers and would expect 6 months's worth of data - relative to the selected period. Yes, I would need to edit interactions, if I add a different slicer that works for my requirement that (hidden) slicer would need to be associated with the original one.