When a gauge hasn't been updated, its value should be reported as 0 rather than its most recent value.
Workaround: use a counter and graph it using nonNegativeDerivative
Alternately, figure out how to clear counters known to be gauges after each Graphite flush, potentially by subclassing Counter and overriding processWith(). A nice side-effect of that is that we'd end up with a concrete type for Gauges and can tell them apart more easily.
metrics 3.0.0 introduces a Gauge type (or so I understand), providing us with the ability to tell counters apart from gauges.