Add tooltip headerFormatter option
I am currently in the middle of a project where I need to format Y values differently per series and the X values need to be converted from timestamps to week numbers.
Right now I'm hacking together a solution based upon feedback in this GitHub issue: https://github.com/highcharts/highcharts/issues/11891
It is not intuitive nor simple. I will work on a PR when time allows.
BrettK shared this idea