For our first client project, I needed to create and compare two timelines - one for planned dates, another one for actual dates. Both planned and actual timelines had multiple milestones with two points each - start and finish. Therefore, I had to create two graphs with dual axes on separate worksheets for each timeline, and then synchronise them across different sheets if I wanted to be able to easily compare them.
First, I created two calculations:
MIN([Planned Startdate],[Actual Startdate])
MAX([Planned Finishdate],[Actual Finishdate])
Then, I created a reference band for each of the timelines, setting up the "Band From" as the MIN calculation, and the "Band To" as the MAX calculation.
Final result - the timeline axes are synchronised in a dashboard as shown below.