Overlay Plot.
If your data looks like this:
x y1 y2
1 10 20
2 22
3 15
4 25 20
5 30
then in Overlay Plot put Y1 and Y2 in the Y box and X in the X box.
If your data looks like this:
x y attribute
1 10 a
2 20 b
3 12 a
4 15 a
5 35 b
6 45 b
7 20 a
then either use Tables > Split to reshape it like the previous data or use it as is in Overlay Plot with Y in the Y box, X in the X box and attribute in the Grouping box. When the plot shows, use the red triangle pull down to change the Overlay Plots item from "no overlay" to "overlay groups".
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