Congratulations to those of you who managed to find the right answer.
Robyn from the Katherine Semar Junior School was one of the first to send in a correct answer:
The shape is a pentagon, the dot is on the middle of the bottom left sloping edge.
Robert and Joel from Cowbridge Comprehensive also sent in correct solutions.
Here is Robert's nice piece of detective work:
Robert goes on to consider on which side of the pentagon the dot must have been placed:
Since the line that marked the lowest speed (which was twice as long as the others) was the last line before the graph repeated itself, the dot had to be on the side that came off the ground last.
Therefore, the shape that made that graph was a pentagon, with the dot in the middle of the lower left side.
Thank you Cristina, from City of London School for Girls, for sending us a very comprehensive solution