Hello,

I need to plot the means of some outcome for two groups (control vs
intervention) over time (discrete) on the same plot, for various subsets
such as gender and grade level. What I have been doing is creating all
possible subsets first, using the aggregate function to create the means
 over time, then plotting the means over time (as a simple line plot with
both control & intervention on one plot) for one subset. I then use par()
and repeat this plot for each gender x grade level subset so they all appear
on one page.


This appears to me to be very similar to an xyplot, something like
 mean(outcome) ~ gender + gradelevel. However, I can't figure out how I
could get both control and intervention lines in the same plot.

Any suggestions? What i'm doing now -works-, but just seems to be the long
way around.

-Robin

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