[R] Looking for easy way to normalize data by groups

[Herbert Jägle]

Hi,

i do have a dataframe representing data from a repeated experiment. PID 
is a subject identifier, Time are timepoints in an experiment which was 
repeated twice. For each subject and all three timepoints there are 2 
sets of four values.

df <- data.frame(PID = c(rep("A", 12), rep("B", 12), rep("C", 12)),
                 Time = rep(c(0, 0, 0, 0, 30, 30, 30, 30, 60, 60, 60, 
60), 3),
                 Dset = rep(c(1, 2),18),
                 Val1 = rnorm(36),
                 Val2 = rnorm(36),
                 Val3 = rnorm(36),
                 Val4 = rnorm(36))

You can plot the data nicely with x=Time and y=Val1 by grouping PID and 
facetting for Dset.

p <- ggplot(df) +
        geom_line(aes(x=Time,y=Val1,group=PID)) +
        geom_point(aes(x=Time,y=Val1,colour=PID)) +
        facet_grid(. ~ Ecc)
    theme_set(theme_bw())
p

I would now like to normalize these data to the mean of the two values 
at Time = 0 for each subject (so having plots in % of the mean Time=0 
value rather than absolute values).

Any suggestions are welcome!
Herbert