graphitePort must point to the port that carbon-cache listens on, not
the graphite webUI port.
With this change I finally got data from statsd to graphite.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
