I'm not sure about the given data but I plotted the scatter plot based on what I understand on how the data should be presented.
Time (years)
3, 6, 9, 12, 15, 18, 21, 24
Number of participants(x1000)
22, 25, 28, 14, 17, 20,15, 9
Attached is the scatter plot created online.
x = time (years)
y = number of participants
