group variables in classes

Dear R users,
I want to group the d values in classes. If I use this script I have a
problem.
classes <- function(x, n){
   s <- seq(0, ceiling(max(x)), by = n)
   factor(n*findInterval(x, s), levels = s)
}
z<-sapply(tapply(t$d,t$plot,function(x) classes(t$d, 4)),table)
z<-cbind(z)

Thank you!
Initial data:
      nr        x      y     d plot plotn xplot yplot
121  162 50.26989 46.230 55.00    1    NA    NA    NA
120  160 43.05703 41.096 12.50    2    NA    NA    NA
1211 162 50.26989 46.230 55.00    2    NA    NA    NA
143  130 48.29959 59.406 42.00    2    NA    NA    NA
144  131 49.56329 55.866 79.50    2    NA    NA    NA
145  161 55.20783 50.046 79.75    2    NA    NA    NA
146  163 55.51600 51.860 40.75    2    NA    NA    NA
147  164 58.28660 52.787 13.25    2    NA    NA    NA
92   200 46.71426 38.114 20.75    3    92    15    95
93   193 56.97232 39.945 77.75    3    93    25    95