Percent rank among values
expr - formula referencing bar data
count - formula specifying a number of bars
expr is calculated for each bar, then results are sorted (ranked) from highest to lowest.
The highest value is given a Rank of 1, the next highest 2, and so on.
Percentile is calculated as 100 * (count - rank + 1) / count.
For the highest value, this will always be 100.
For the lowest value, this will always be 100 / count (percentile can never be 0).