pmwebd graphite pngs: tweak y axis labeling
pmwebd graphite pngs: tweak y axis labeling

Previously, a hard-coded 0.5 epsilon squashed small near-zero time
series (seen on rate-converted counters) to wiggles, due to poor
vertical autoscaling.  Now we choose a relative epsilon that is
informed by the visual space / precision alotted to the drawn y axis
label (5 digits + exponent).  Eyeballing on a range of time series,
both near and far zero now results in more eye-pleasing graphs.  QA
not impacted, as these are purely visual artifacts.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp