You can bring you data in A:C to the format

X       
1   20  Y1
2   20  Y1
3   20  Y1
5   25  Y1
10  30  Y1
1   25  Y2
3   42  Y2
6   50  Y2
8   50  Y2
10  50  Y2

and then use

=QUERY(A:C,"select A, sum(B) where A is not null group by A pivot C")

to generate

X   Y1  Y2
1   20  25
2   20  
3   20  42
5   25  
6       50
8       50
10  30  50

ready to be plotted.

You can bring you data in A:C to the format

X       
1   20  Y1
2   20  Y1
3   20  Y1
5   25  Y1
10  30  Y1
1   25  Y2
3   42  Y2
6   50  Y2
8   50  Y2
10  50  Y2

and then use

=QUERY(A:C,"select A, avg(B) where A is not null group by A pivot C")

to generate

X   Y1  Y2
1   20  25
2   20  
3   20  42
5   25  
6       50
8       50
10  30  50

ready to plot.