Scatter plot does not show all data

Hi all, I made a scatter plot with date in x and cumulative total for student's credit in Y axis. StudentId in the legend. There are 20 students enrolled in every year and they are studying minimum of 5 years to 9 years. The plot does not show all students. When I choose some StudentIDs for example 20 in total, All are shown in the plot.

I have this problem also in line chart but when density fiture is turns on it shows all lines but it's such a mess.

Could you please help me to get better insight into my data? How could I group students in a meaningfull way? Is it necessary to make the groups to reduce data?

Thanks in advance.