On Route planner, weather conditions are changing along the route according to the time.
The diagram shows the conditions that you will encounter during the trip, "on one screen without having to mouse along the timeline". Or do you get a bug?
The map, of course, shows the weather as forecast for each position of the boat.
At least could you show a screenshot ?
Capture d’écran 2020-09-28 à 10.37.20.png