@idefix37 Thanks a lot for looking into it !
I tried something different : importing my gpx file in the Flight planner plugin !!! and it kinda works : I do have to set an average speed which is not ideal but apart from that, the weather map changes with the position along the route.

In the gpx file I'd like to display, for each point I have a timestamp and a position (lat/long). There is no speed in the file.
But you are right that if you were to calculate the speed, it would be highly variable, anywhere btw 0 and 30kts.