[R] Help with scatter plot

[A Ezhil]

Dear All,

I would like to add the mean line in the scatter plot by groups. I have plotted (X.val is 3 groups: 0, 1, 2): plot( Y ~ X.val). How can I add mean/median line (horizontal) for each group on the scatter plot?

Thanks in advance.

Kind regards,
Ezhil