I read the docs a million times but I can't figure out what it does. What does nPercentile do in Graphite and how it differs from percentileOfSeries?
nPercentile: "Returns n-percent of each series in the seriesList."
This converts every series you give it to a single value*, that is the n-th percentile of that series (but it will output as many series as it was given as input).
*note that as everything in graphite, this single value will be returned as a series, containing many times (as many as needed to fill the requested time-range) the same single value.
percentileOfSeries: "percentileOfSeries returns a single series which is composed of the n-percentile values taken across a wildcard series at each point."
This returns a single series, which, for every timepoint, contains the n-th percentile of the different input series.