Provide the rank of values within each group.
average: average rank of group.
min: lowest rank in group.
max: highest rank in group.
first: ranks assigned in order they appear in the array.
dense: like ‘min’, but rank always increases by 1 between groups.
False for ranks by high (1) to low (N).
keep: leave NA values where they are.
top: smallest rank if ascending.
bottom: smallest rank if descending.
Compute percentage rank of data within each group.
The axis of the object over which to compute the rank.