Be aware that the object that aggregate returns with bar() is more complicated
than you think.
str(aggregate(iris$Sepal.Length, by = list(iris$Species), FUN = bar))
'data.frame': 3 obs. of 2 variables:
$ Group.1: Factor w/ 3 levels "setosa","versicolor",..: 1 2 3
$ x : num [1:3, 1:2] 5.006 5.936 6.588 0.352 0.516 ...
..- attr(*, "dimnames")=List of 2
.. ..$ : NULL
.. ..$ : chr "mean" "sd"
So you get a two column data.frame whose second column is a matrix.
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