I recommend the ancova function in the HH package.

install.packages("HH")
library(HH)
example(ancova)
?ancova

For your example,

tmp <- textConnection(
"day     A       B
0       10.0    10.0
7       9.0     9.1
14      8.0     8.2
21      7.0     7.3
28      6.0     6.4
35      5.0     5.5
42      4.0     4.5
49      3.0     3.6
56      2.0     2.7")
anco <- read.table(tmp, header=TRUE)
close.connection(tmp)

wind <- data.frame(day=rep(anco$day, 2),
                   y=c(anco$A, anco$B),
                   type=rep(c("A","B"), each=9))
ancova(y ~ day + type, data=wind)

Rich

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