Error when graphite returns only one value

Hi,
two small changes. Don't know if they make sense.
First use /usr/bin/env python, so the script works with virtualenv
Second, i had my graphite return metrics with only one datapoint in the requested time range, then the script fails, with "Graphite returned an empty list of values". I wonder if the construct in line 91 is anyhow useful.

I send you the pull request, because datacratic stopped supporting this script.

Best regards!