As discussed already, using blend and keeping line stable is 100% depends on primary dates has all dates or not.
If not, there is no way to keep line stable with real data.
As I proposed, using "Picture" instead is only the way I know of and it' exactly same concept with reversing the X-axis.
Screen capture and save the picture in some folder,
background picture range and X,Y axis range needs to be matched exactly. attach workboon if still struggling.