To get the main anti-diagonal, you can do this. (rev to get stated order)
rev
mydata[row(mydata) + col(mydata) == ncol(mydata) + 1]
You can get the diagonal below that with
mydata[row(mydata) + col(mydata) == ncol(mydata) + 2]
and so on
you can try to get the antidiagonal by
diag(mydata[,nrow(mydata):1])[1] 65 90 55
or
diag(as.matrix(rev(as.data.frame(mydata))))
You can get that diagonal by flipping your matrix before calling diag:
diag
diag(mydata[ , 3:1]) # [1] 65 90 55
Doing mydata[ , 3:1] is indexing the columns of your matrix in reverse order.
mydata[ , 3:1]