My records are observations of organisms and the time when they were seen (hh:nn:ss). I would like to group my records into whole minute intervals and summarize the number of unique (different) organisms per minute. Ultimately, I am trying to produce a graph that shows the number of minutes it takes until no more unique organism are observed. The purpose of this graph is determine the amount of sampling time required before all of the biodiversity present in the ecosystem has been observed. Thanks