The ChickWeight data is divided by the mean weight of all chicks in the respective times using the aggregate function.

chick_avgdiet <- aggregate(list(Weight=ChickWeight$weight),
     by=c(list(Diet=ChickWeight$Diet), list(Time=ChickWeight$Time)),
     FUN=mean)
head(chick_avgdiet)

##   Diet Time Weight
## 1    1    0  41.40
## 2    2    0  40.70
## 3    3    0  40.80
## 4    4    0  41.00
## 5    1    2  47.25
## 6    2    2  49.40