Hi jeanfloriani,
Thanks for contacting us with your question.
Please provide me with a live demo showing what you have achieved so far. You can start here:
https://jsfiddle.net/BlackLabel/Lcf36abd/
Looking at the attached image I can tell that there are two series and to create such chart you would need two sets of data, one for timeline and second one with average values for that dashed line in the middle. Highcharts doesn't calculate it automatically.
Regards!