Data details in Appendix.

head(Orange)

##   Tree  age circumference
## 1    1  118            30
## 2    1  484            58
## 3    1  664            87
## 4    1 1004           115
## 5    1 1231           120
## 6    1 1372           142

dim(Orange)

## [1] 35  3

table(Orange$Tree)

## 
## 3 1 5 2 4 
## 7 7 7 7 7

str(Orange)

Classes 'nfnGroupedData', 'nfGroupedData', 'groupedData' and 'data.frame':  35 obs. of  3 variables:
 $ Tree         : Ord.factor w/ 5 levels "3"<"1"<"5"<"2"<..: 2 2 2 2 2 2 2 4 4 4 ...
 $ age          : num  118 484 664 1004 1231 ...
 $ circumference: num  30 58 87 115 120 142 145 33 69 111 ...
 - attr(*, "formula")=Class 'formula'  language circumference ~ age | Tree
  .. ..- attr(*, ".Environment")=<environment: R_EmptyEnv> 
 - attr(*, "labels")=List of 2
  ..$ x: chr "Time since December 31, 1968"
  ..$ y: chr "Trunk circumference"
 - attr(*, "units")=List of 2
  ..$ x: chr "(days)"
  ..$ y: chr "(mm)"