groupByNodes() does not accept negative values for node offsets, but should

[markzabaro]

Expected: Based on https://graphite.readthedocs.io/en/latest/functions.html#graphite.render.functions.groupByNodes and many other functions' behavior, I assumed that positive and negative 0-indexed offsets should work.

Observed:

• This gives an internal server error: /render/?from=-1months&width=1300&height=600&target=groupByNodes(this.is.a.very.long.timeseries.path.and.it.might.be.a.little.easier.to.count.from.the.end.instead.of.the.beginning,"sum",-10)
• This renders a metric/render/?from=-1months&width=1300&height=600&target=groupByNodes(this.is.a.very.long.timeseries.path.and.it.might.be.a.little.easier.to.count.from.the.end.instead.of.the.beginning,"sum",14)

Anonymized to protect confidential path names. :)